Diseases and conditions that impact maternal and fetal health and the potential for nanomedicine therapies

Abstract

Maternal mortality rates in the United States have steadily increased since 1987 to the current rate of over 16 deaths per 100,000 live births. Whereas most of these deaths are related to an underlying condition, such as cardiovascular disease, many pregnant women die from diseases that emerge as a consequence of pregnancy. Both pre-existing and emergent diseases and conditions are difficult to treat in pregnant women because of the potential harmful effects of the treatment on the developing fetus. Often the health of the woman and the health of the baby are at odds and must be weighed against each other when medical treatment is needed, frequently leading to iatrogenic preterm birth. However, the use of engineered nanomedicines has the potential to fill the treatment gap for pregnant women. This review describes several conditions that may afflict pregnant women and fetuses and introduces how engineered nanomedicines may be used to treat these illnesses. Although the field of maternal-fetal nanomedicine is in its infancy, with additional research and development, engineered nanotherapeutics may greatly improve outcomes for pregnant women and their offspring in the future.

1. Introduction

1.1. Advantages of nanomedicine for maternal-fetal health and barriers they must overcome

Clinicians and researchers have increasingly shown interest in developing new strategies to ensure maternal and fetal health during pregnancy, as complications that arise during pregnancy are difficult to treat with traditional therapeutics. A major limitation of most therapies is that they fail to specifically target the maternal compartment, fetal compartment, or placental interface as necessary for the given condition, which reduces efficacy and increases detrimental side effects. The field of nanomedicine has grown substantially over the last two decades and is focused on developing innovative tools that offer site-specific delivery capabilities (Fig. 1). A majority of nanomedicine research has been devoted to targeting tumors [1,2], but lessons learned from these studies may be applied to maternal-fetal health to improve the treatment of pregnancy complications.

Fig. 1.

Types of engineered nanomedicines and their potential to advance maternal-fetal health. A plethora of engineered nanomedicines have been developed to enhance cargo delivery to specific sites and thereby improve therapeutic ratios. These tools may be utilized in the future to improve maternal and fetal health during pregnancy by enabling precise cargo delivery to the mother, placenta, or fetus following systemic or vaginal administration.

In this review, engineered nanomedicines are defined as nanoscale systems that are well controlled and designed for a specific medicinal purpose. Engineered nanomedicines include, but are not limited to, polymeric nanoparticles, liposomes, dendrimers, peptide-drug conjugates or viral vectors (Fig. 1). These systems have the advantage of being easily modifiable and targetable through surface conjugation techniques, and tunable in terms of size, shape and surface chemistry by tailoring the synthesis parameters. Additionally, these systems can be designed with specific properties and materials to ensure safety and biocompatibility, as has been demonstrated in several in vitro and preclinical studies [3–10]. Maternal-fetal medicine could greatly benefit from the introduction of engineered nanomedicines to the field.

In order for engineered nanomedicines to effectively treat maternal-fetal health conditions, they must overcome distinct biological barriers that depend on the route of administration. For example, systemically administered nanomedicines must minimize hepatic clearance and off-target effects, while vaginally administered nanomedicines must penetrate cervicovaginal mucus (CVM). Studies in the broad field of nanomedicine have shown that modifying the surface of nanoparticles with poly(ethylene glycol) (PEG), zwitterionic materials, or biological membranes [11] can be an effective strategy to increase circulation time in the blood and reduce hepatic clearance [12]. PEG is the most widely utilized nanoparticle coating, and it acts as a hydrophilic shell that repels opsonins and other serum proteins from adsorbing to the nanomedicine and tagging it for clearance by macrophages [12]. Researchers developing nanomedicines for maternal-fetal health applications could capitalize on this knowledge to extend the circulation time of nanomedicines in pregnant patients following systemic administration. Likewise, studies have shown that the size and shape of nanomedicines play a large role in biodistribution and clearance [13], and this can be exploited in maternal-fetal medicine. Finally, the use of targeting methods, such as conjugating specific peptides or antibodies to a nanomedicine to enhance site-specific retention has been well established in the field, and this approach may be used to enhance the delivery of nanomedicines to the mother, fetus, or placenta as necessary for a specific indication. It is imperative that nanomedicines administered to pregnant women arrive at the appropriate location, as off-target delivery may not only negatively affect the mother, but can also cause life-long morbidity or mortality to the fetus.

Notably, many of the strategies used to enhance the safety and efficacy of systemically delivered nanomedicines may also benefit vaginally administered nanomedicines. CVM is a protective barrier that lines the vaginal epithelium and removes bacteria and antigens from the body in the form of discharge [14,15]. Due to changes in progesterone and estrogen, the viscosity and presence of CVM increases with gestational age. Studies have shown that nanoparticles coated with PEG or other agents that impart a neutral charge can penetrate CVM more effectively than non-modified nanoparticles because of decreased electrostatic interactions [16,17]. Further, nanoparticles less than 200 nm in hydrodynamic diameter can penetrate CVM better than larger nanoparticles because they can escape the fibrous mucin networks that make up the CVM [15,18]. The size, material properties, and surface chemistry of engineered nanomedicines are important features that can be turned to enable them to overcome biological barriers that hinder successful treatment of pregnancy complications.

Overall, the ability to tune nanomedicine transport across biological barriers makes them advantageous relative to freely delivered therapeutic agents. By tailoring nanomedicines’ design characteristics to enhance or reduce cargo delivery to specific sites after systemic or vaginal delivery their efficacy and safety can be regulated. For example, although small molecule drugs and biologics cannot surpass CVM and are quickly cleared following vaginal administration, some nanoparticles have been shown to escape this barrier and reach the desired targets [19–21]. Thus, this local delivery route may be harnessed in the future as a means to deliver medicines directly to maternal reproductive organs while avoiding systemic vasculature and hepatic clearance. This may reduce potential risks for pregnant mothers. Likewise, nanomedicines delivered systemically are advantageous over freely delivered agents because the relatively larger size of the nano-delivery system may reduce drug transport across the placenta, minimizing potential harm to the fetus. In sum, the tailorability of nanomedicines gives them great potential to overcome the barriers and challenges associated with treatment of pregnancy complications.

Within maternal-fetal medicine, there are numerous diseases and conditions that may be addressed through nanomedicine. This review will discuss several of these challenges in detail to ignite innovation by nanomedicine researchers. In Section 2, we briefly introduce the physiologic changes that occur during pregnancy and discuss models of pregnancy that researchers may utilize when developing nanomedicines for the treatment of pregnant women. In Sections 3 and 4, we delve more deeply into the use of nanomedicine to treat maternal and fetal conditions during pregnancy. Finally, we discuss other pregnancy conditions that have yet to be investigated and how future nanomedicine research can be used to advance treatment options.

2. Physiologic changes that occur during pregnancy and preclinical models of pregnancy that are available to nanomedicine researchers

2.1. Pregnancy alters maternal physiology

Pregnancy alters physiologic homeostatic conditions, usually without detrimental effects to the mother. Nanomedicine researchers should appreciate these changes as they could impact drug efficacy and function. One notable change is that the volume of maternal blood increases by 40% by the end of pregnancy compared to volumes prior to pregnancy [22]. However, the increase in red blood cell mass is not proportional, so there is a decrease in hemoglobin concentration, hematocrit, and red blood cell count [23]. This means that systemically administered nanomedicines will interact with blood components at a different rate than they would in a non-pregnant patient. The maternal heart rate and cardiac output also increase during pregnancy, but blood pressure does not typically increase due to decreased resistance in the vasculature as the vasculature is desensitized to vasoconstrictive molecules [24]. In fact, blood pressure decreases in the first and seconds trimesters [23]. It remains to be thoroughly investigated how the altered blood pressures during pregnancy impact nanomedicine transport through vasculature. Another physiologic change that occurs during pregnancy is that maternal metabolism switches from using mostly glucose as an energy source to using lipids, which allows glucose to be an energy source for the fetus [24]. Almost every organ system experiences some physiological change during pregnancy [23], and these changes often do not impact the woman’s health status. However, these changes can worsen or uncover disease states that the woman already suffers from, like asthma or diabetes (Fig. 2). Nanomedicine may offer the ability to treat such conditions that are exacerbated during pregnancy.

Fig. 2.

Maternal and fetal conditions that are exacerbated during pregnancy or that are induced by pregnancy (as indicated by *) (Diabetes can be pre-existing or induced by pregnancy,^#). FGR, fetal growth restriction; PPROM, preterm premature rupture of membranes.

In addition to maternal changes, the development of the fetus can be complicated by a variety of diseases (Fig. 2). Unsurprisingly, the health of the mother can largely influence the health of the fetus. In addition, the function of the placenta is imperative for proper fetal development as issues of oxygen and nutrient transport to the fetus could have dire consequences. Nanomedicines delivered systemically must be able to navigate to or across the placenta to treat many fetal conditions. Fetal development can also be disrupted by causes besides a dysfunctional placenta, such as inherent genetic disorders, congenital malformations, or maternal infections. In fact, maternal anemia can reduce fetal iron levels that are critically needed for neurodevelopment which is associated with fetal neurodevelopment disorders [25]. Many factors are involved in maintaining healthy fetal development that can impact the baby into their adult life. Nanomedicines designed to treat fetal conditions must be designed to target the appropriate factor.

Organogenesis is largely completed by the end of embryonic development through the first trimester (Carnegie Stage 23), while cellular growth and terminal differentiation continues in the second and third trimesters [26]. These processes are vastly different in terms of mechanism and can therefore be affected differently by exogenous stimuli, like drugs. Generally, the embryo/fetus is most susceptible to fatal developmental disruptions in the first trimester, as correct organogenesis is vital for a viable baby [27]. While the ethical concerns regarding fetal health are essential in treatment selection, they do result in pregnant women having to make difficult choices between their health and their fetus’ health. There is a large gap in knowledge regarding the effect of many drugs on pregnancy and how women requiring maintenance medications could safely be treated. These women could benefit greatly from the development of effective nanomedicines that do not hinder fetal development.

2.2. Models for the study of maternal-fetal health

In order to study nanomedicine applications in maternal-fetal health, the correct model must be chosen for the specific questions that are being asked. In depth discussion of animal [28–30] and in vitro [31] pregnancy or placenta models can be found elsewhere [32], but brief synopsis is provided here. In vitro models, such as a transwell [33] or microfluidic [34] systems, are inherently reductionist, but can allow for specific studies under tight control that may be ideal for understanding the mechanism of drug transport across the placenta. For more holistic understanding of potential responses to therapeutics, animal models may be preferred. Different animals have different placenta structures, with non-human primates and rodents having the most similar placental structures to humans [29]; therefore, these models are the most widely used in the field but the differences in placentation must be recognized [35]. Of rodents, mice are most commonly used due to the genetic control and short gestational period, however, guinea pigs have placenta structures more similar to humans and a more similar immune system which may be of importance for some studies [36]. One note to consider is that mice, the preclinical model of choice for many nanomedicine in vivo studies, do not develop a cervical mucus plug as humans do in pregnancy. While large animals generally have more similar physiology to humans, which may be necessary to understand the effect of a therapeutic on the maternal or fetal systems [28], their high cost makes small animals models more common for initial evaluation and safety studies. Proper considerations must be taken when choosing a model to ensure the results obtained are appropriate for the intended studies.

In sum, the altered and dynamic physiology of pregnancy, and the numerous organs involved, increases the risk for something to go awry. However, there are also limited ways to treat conditions and diseases during pregnancy to ensure safety to the fetus and mother. There is an excellent opportunity for engineered nanomedicines in the maternal-fetal health space as these vehicles can be designed and developed in a tailorable manor to achieve specific goals. Engineered nanomedicines can be formulated with tunable size, shape, material, and surface properties to facilitate passive or active targeting and controlled material degradation and cargo release kinetics, allowing them to be applied in a variety of settings. In order for engineered nanomedicines to be appropriately designed, the physiology of the disease and patient condition must be understood. With the correct background and knowledge, as well as use of the appropriate model, engineered nanomedicines can be developed to fill the treatment gap established by pregnancy to treat women without risking excess harm to the fetus.

3. Maternal conditions that may benefit from nanomedicine

Pregnancy complications that initially affect the mother’s health subsequently affect the fetus. In this review, we define maternal conditions as those with long-term effects on the mother, and a similar definition applies for fetal conditions. We also discuss conditions that may be exacerbated or pose several risks to the mother’s health with the onset of pregnancy. In each section we discuss the disease pathology as well as the use of nanomedicines to resolve symptoms for each condition. Table 1 summarizes some of the nanomedicine approaches discussed in this section.

Table 1.

Representative studies exploring the use of nanomedicine to treat maternal conditions during pregnancy.

Disease	Nanomedicine	Therapeutic action	Model	Outcome	Reference
Diabetes	Cerium oxide nanoparticle	Decrease ROS	Pregnant diabetic mouse	Increased embryonic weight, decreased ROS, no change in maternal blood glucose levels	[62]
Hypertension	Fusion protein	Inhibit pan-C3 convertase	BPH/5 pregnant mouse	Increased placental weight and normalized morphology, increased VEGF	[79]
Preeclampsia	Fusion protein	Inhibit pan-C3 convertase	CBA/J x DBA/2 preeclampsia mouse model	Prevented oxidative stress, lowered sFlt-1, improved kidney function	[81]
	Fusion protein	Sequester circulating sFlt-1	Reduced Uterine Perfusion Pressure (RUPP) rat model	Decreased free levels of sFlt-1, decreased maternal blood pressure and NO levels	[90]
	Dendrimer siRNA complex	Inhibit sFlt-1 expression	TNF-α rat model	Reduced hypertension, proteinurea, and circulating sFlt-1. Increased fetal weight	[94]
	Cholesterol siRNA complex	Inhibit sFlt-1 expression	Healthy pregnant mouse or uteroplacental ischemia baboon	Mouse: reduced sFLT1 mRNA levels in placenta, no change in pup weight Baboon: reduced free sFlt-1, blood pressure, proteinuria, no change in newborn weight	[95]
	Lipid-polymer siRNA nanoparticle	Inhibit sFlt-1 expression	Healthy pregnant mouse	Decreased sFLT1 mRNA and free sFlt-1, fetal weight unchanged	[96]
Ectopic Pregnancy	PLGA/lecithin/PEG nanoparticle	Reduce fetal and placental growth with methotrexate	Healthy pregnant mouse	Less than 10% fetal survival	[101]
Choriocarcinoma	Bacterial derived nanosphere	Slow tumor growth with doxorubicin	Mouse JEG-3 tumor xenograft	Slowed growth of tumor compared to control	[99]
	PLGA/lecithin/PEG nanoparticle	Slow tumor growth with doxorubicin	Mouse JEG-3 tumor xenograft	Slowed and reduced tumor growth, reduced death rate	[100]

3.1. Bacterial vaginosis

A disruption of the vaginal microbiome, bacterial vaginosis (BV), can cause many challenges in nongravid as well as gravid women. Lactobacillus species dominate the vaginal microbiome in healthy conditions. They secrete lactic acid, as well as other anti-microbial molecules, that maintain the vaginal pH at 3.5–4.5 and reduce the abundance of pathogenic bacteria [37]. In BV, the abundance of Lactobacillus species decreases and is replaced by a variety of pathogenic bacteria, such as E. coli and G. vaginalis [38]. This imbalance is associated with vaginal discharge, pain, infertility, preterm birth, and increased susceptibility to other infections like HIV and urinary tract infections [39–41]. BV affects 30% of women during childbearing ages [38] but because symptoms are mild and often undetected, the onset of pregnancy can pose dangerous risks to both the mother and fetus. About 63% of pregnant patients with BV are asymptomatic and do not exhibit changes in vaginal discharge color or odor [42]. As a result, pregnant women have an increased risk of spontaneous abortion, preterm birth (PTB) [40,43,44] and preterm premature rupture of membranes (PPROM) [38,42,45–47]. While not life threatening for a woman, babies delivered by women with BV have an increased risk of assisted ventilation or respiratory distress at birth and sepsis [48].

Current treatments for BV patients include oral antibiotics and vaginal creams. However, 10–15% of patients do not improve after the first course of antibiotics [38] and recurrence is significant within 6 months of treatment [49]. This is in part due to that Lactobacilli growth is inhibited by antibiotic treatments. Thus, women with BV face a heightened risk of acquiring sexually transmitted diseases [38]. Due to the severity of this condition, there is an apparent need for BV treatment in pregnant women.

Current studies have developed probiotic treatments and nanoparticle-based systems to combat BV in non-pregnant women. Unlike the antibiotic methods previously described, these systems have evaluated Lactobacilli regrowth and inflammatory responses in mice following BV infection. Estrogen-treated mice were infected with E. coli for 5 days prior to treatment [50] and a single high or low dose of Clostridium butyricum was delivered vaginally once per day for 6 days. This probiotic treatment was compared to an antibiotic treatment, kanamycin [50]. Researchers conducting this study observed a reduction in neutrophil presence and bacteria count for the high dose C. butyricum treatment. Only the high dose treatment restored lactobacillus growth, indicating this probiotic inhibits E. coli growth and promotes lactobacillus growth [50]. While this study delivered free drug, nanoparticle-based systems encapsulating probiotic drugs would enhance the current treatment strategy as the nanoparticles would increase drug retention and subsequently lactobacillus growth.

Although limited studies have explored the use of drug-loaded nanocarriers to treat BV, such systems have been used previously in the treatment of HIV. For example, one study developed a poly(lactic-co-glycolic acid) (PLGA) nanoparticle film loaded with an HIV antiviral drug, tenofovir, and explored its effects in non-pregnant CD-1 mice. PLGA nanoparticles (PLGA-NPs) loaded with tenofovir were embedded in polyvinyl alcohol (PVA) and hydroxypropyl methyl cellulose (HPMC) to create a dissolvable vaginal film [51]. Drug release profiles in vitro displayed 80% drug release within 5–10 min and 90% release within the first 2 h. Films were placed in the vagina once daily for 14 days in diestrus phase mice and the animals were evaluated for cytokine production, toxicity levels, and morphological changes. Data from this study revealed mild thinning of the vaginal epithelium and slightly raised IL-1β levels in the film treated group [51]. No histological changes in the cervix or uterine horns were observed between groups and films displayed minimal toxicity levels in vivo [51]. Although this study did not evaluate BV inhibition, it confirmed PLGA-NP vaginal films have minimal toxicity effects in vivo. Future treatment methods should modify the release kinetics of the film to target BV infections and recurrences during pregnancy. Another potential nanomedicine would be PEG-coated nanoparticles encapsulating BV antibiotics, such as clindamycin and metronidazole. This approach could be beneficial versus existing antibiotic regimens because of PEG-coated nanoparticles’ demonstrated ability to penetrate the CVM barrier. Considering previous studies have not evaluated the toxicity of antibiotics or probiotics in pregnant animal models, future nanoparticle research should determine how either drug classification might affect fetal development and maternal health.

3.2. Diabetes Mellitus

During pregnancy women’s sensitivity to insulin changes. Relatively higher levels of estrogen increase insulin sensitivity, while later in pregnancy, production of lactogen and progesterone from the placenta decrease insulin sensitivity [52]. These physiologic changes make diabetes mellitus a risk factor for pregnancy complications. Maternal hyperglycemia induces fetal hyperinsulinemia and results in excess fetal growth that can increase complications during delivery or even cause late-term death [53]. Pregnant women with type 1 diabetes have increased rates of preeclampsia, premature delivery, mortality, and giving birth to a child with congenital malformations [54]. There has also been an increase in pregnancy induced diabetes, gestational diabetes mellitus (GDM), as the rate of type 2 diabetes and risk factors, like obesity, increase [55]. GDM affects up to 14% of all pregnancies in the United States and is responsible for almost 90% of pregnancies complicated by diabetes [56]. Women with GDM are at an increased risk of remaining diabetic, being diagnosed with type 2 diabetes in the future [57], and their children have an increased risk of obesity and impaired intellectual achievement [58,59]. The most common symptom of GDM in the first trimester is hypertension, which can lead to an increased risk of developing preeclampsia (Section 3.4) or preterm delivery.

The current management methods for GDM are either diet or medication. Women diagnosed with GDM who exhibit low risk factors for additional pregnancy complications are provided nutritional guidance to limit carbohydrate intake and increase aerobic exercise [60]. Women with high risk factors are given incremental oral metformin or glyburide prescriptions, with increased doses to address rising blood sugars [60]. Neither of these hypoglycemic agents are FDA approved but have shown minimal increased perinatal complications when compared to insulin [55,60,61]. Newer treatments have suggested antioxidants such as vitamins D and E or zinc can alleviate oxidative stress caused by GDM, but these studies have not assessed effects on fetal development following treatment [55,62].

Few researchers have used nanomedicine-based therapies to combat GDM, although one study used cerium oxide nanoparticles (nanoceria) as an antioxidant treatment in pregnant mice. The synthesis of nanoceria formed a crystalline structure with antioxidant properties similar to vitamin E. This formulation was shown to protect against reactive oxygen species (ROS) that cause oxidative stress by shifting between oxidation states [63]. Pregnant diabetic mice were given a single dose of either nanoceria or vitamin E every day for 16 days. Weight changes, blood glucose levels, ROS formation, and embryonic developmental effects were evaluated. It was determined that nanoceria increased embryonic weight and decreased morphological embryonic changes [63]. Further, nanoceria decreased ROS formation and displayed a stronger antioxidant effect than the vitamin E treatment. However, nanoceria did not significantly change maternal blood glucose levels as would be anticipated [63]. The primary outcome of nanomedicine to treat diabetes must be to lower blood glucose levels; while this has not been accomplished in pregnancy models, it has been shown in non-pregnant animal models [64–67] which may be able to be translated to pregnancy models. In particular, glucose-responsive insulin delivery systems such as glucose-sensitive polymeric nanogels [68], pH-sensitive polymeric and metal-organic framework (MOF) [69,70] nanoparticles, and charge-switchable polymer conjugates [71] have shown promise when used for subcutaneous or transdermal insulin delivery in non-pregnant murine and swine models of diabetes [68–71]. Glucose-responsive nanomedicines are especially good candidates for applications in the maternal-fetal health space as they reduce the risk of hypoglycemia, a common side-effect of many diabetic therapies, which is dangerous for the fetus [72]. These glucose-responsive materials only release insulin when there is an excess of glucose, which better mimics the natural healthy physiologic response to hyperglycemia and may allow the mother to better maintain blood glucose levels. Such strategies may be beneficial in treating diabetes during pregnancy and this should be explored in future work.

3.3. Hypertension

Chronic hypertension is a common disease that impacts up to 5–10% of pregnant women [73]. Hypertension during pregnancy poses increased risks such as maternal pulmonary edema and acute renal failure and stillbirth, premature delivery, and small for gestational age infants [74]. Chronic hypertension is also a high risk factor for preeclampsia (Section 3.4). Treatment of chronic hypertension and pregnancy induced hypertension disorders, like preeclampsia, is complicated by the potential effects of a therapeutic on the fetus compared to the need of treatment. It may be better to clinically manage mildly hypertensive women with diet and lifestyle changes instead of treatment with a pharmacological agent due to the potential risk to the fetus [74,75]. However, once maternal blood pressure is elevated above critical levels, pharmacological interventions are needed. There is limited information and knowledge about the potential teratogenicity of many anti-hypertensive therapies, so monitoring of patients is critically important during therapy.

Pre-existing hypertension may cause a variety of different pregnancy complications. A BPH/5, mildly hypertensive mouse has pregnancies characterized with symptoms similar to human. These mice spontaneously have fetal growth restriction, abnormal placentation, and defects in maternal decidual arteries [76,77]. These issues lead to inflammation and activation of the complement pathway, which has been shown to be associated with adverse pregnancy outcomes [78–80]. Complement inhibition using a fusion protein (CR2-Crry) between a pan-C3 convertase inhibitor, Crry, and a targeting protein that binds to C3 degradation products, CR2, was able to increase placental weight, normalize junctional zone and spiral artery morphology, increase placental vascular endothelial growth factor (VEGF) concentration, and reduce neutrophil recruitment to the placenta without altering the numbers of uterine natural killer cells or macrophages [81]. These results indicate that the complement pathway is part of the pathology of adverse outcomes during pregnancy due to hypertension. As hypertension is a risk factor for preeclampsia, the complement pathway is also suggested to be part of the pathology of that condition as well [78,80,82]. In a CBA/J x DBA/2 mouse model of preeclampsia the delivery of CR2-Crry was able to prevent oxidative stress, lower circulating sFlt-1 (soluble fms-like tyrosine kinase-1) levels, reduce fetal resorption, and improve kidney function [83]. Success using CR2-Crry to treat both hypertension and preeclampsia in different mouse models shows how closely related the two conditions are. It also provides insight to potential targets for nanomedicine to treat hypertension and/or preeclampsia.

There are several mouse models that have been developed to study hypertension and preeclampsia. This enables excellent nanomedicine-based research to be completed as many different causes of the diseases can be specifically studied and treated in preclinical settings. There are specific transgenic mouse models that allow for research into differing causes of hypertension, like the renin angiotensin system (RAS), endothelial nitric oxide synthase (eNOS), or endothelin-1 (ET-1) [84]. These models allow for discreet, target specific studies into the treatment of hypertension via one mechanism with nanomedicine. However, hypertension is a condition that develops from many factors, therefore more general models of hypertension, such as spontaneously hypertensive mice or rats, allow for a more complete depiction of human disease and therapeutic response. Hypertension in these animals is less controlled making the results gained from them harder to interpret [84]. Regardless of the animal model chosen, nanomedicine allows for controlled delivery and targeting of current drugs that are contraindicated with pregnancy, and the use of these animal models will enable proper studies to confirm the function of novel therapeutics and delivery systems.

3.4. Preeclampsia

Preeclampsia (PE) is the development of severe hypertension and proteinuria or other maternal organ dysfunction after 20 weeks gestation [85]. PE can lead to severe maternal and fetal consequences such as seizures and death of mother and baby [74,75,85]. PE is thought to stem from a dysfunctional placenta, although the exact etiology of the disease is unknown. The lack of knowledge regarding PE onset has made treatment difficult and focused on lessening symptoms rather than aimed towards treating the underlying cause. However, some studies have been able to treat PE in animal models, suggesting nanomedicine may have a substantial impact on PE management in the future.

The finding that pregnancies complicated by PE exhibit increased maternal sFlt-1 [86] opened the door to a potential new target for treatment of PE. It was also shown in this study that increased levels of sFlt-1 are associated with decreased maternal free VEGF and placental growth factor (PlGF), leading to the hypothesis that the excess sFlt-1 could bind and sequester free VEGF and PlGF in the maternal blood, thereby reducing the pro-angiogenic function of these growth factors, resulting in endothelial dysfunction, hypertension, and proteinuria [86]. Since this initial report, many studies have shown that delivery of recombinant VEGF or the VEGF gene can reduce symptoms of PE in multiple different rat and mouse models [87–91]. The effect could come from either the delivered VEGF acting physiologically in its pro-angiogenic function or by the delivered VEGF binding to excess sFlt-1, leaving endogenous VEGF free to act on downstream targets. More recently, an elastin like peptide (ELP)-VEGF conjugate has been used to actively sequester excess sFlt-1 from the maternal circulation while ensuring VEGF cannot cross the placenta and potentially disrupt fetal development [92]. The ELP-VEGF conjugate increased total sFlt-1 amount, but decreased levels of free sFlt-1 showing that the conjugate successfully bound excess free sFlt-1 and decreased maternal blood pressure and nitric oxide levels [92]. Subsequent work showed that changing the molecular weight of the ELP can provide control over the biodistribution and clearance times of the ELP conjugates [93]. Increasing ELP size yields slower plasma clearance but increases organ deposition [93]. This is an important fundamental finding that may guide nanomedicine researchers in developing other drug conjugate systems to treat PE.

There has also been interest in reducing sFlt-1 expression instead of inactivating circulating s-Flt-1. To achieve this, small interfering RNA (siRNA) against sFlt-1 was incorporated into different delivery vehicles. Poly(amidoamine) (PAMAM) dendrimers have been shown to be biocompatible and have low transport rates across the placenta [94], making them intriguing carriers for treating pregnancy complications. By taking advantage of electrostatic forces between PAMAM free amines and phosphate groups in siRNA, self-assembled si-sFlt1-PAMAM complexes can be formed. Using a tumor necrosis factor-α (TNF- α) induced PE rat model [95] si-sFlt1-PAMAM was shown to reduce hypertension, proteinuria, and circulating sFlt-1 [96]. The siRNA delivery system was also able to increase fetal weight without having any detected maternal tissue damage or fetal resorptions [96]. Beyond this dendrimer system, hydrophobically modified asymmetric siRNAs conjugated to cholesterol (hsiRNAs) have also been used to target specific isoforms of sFlt-1, namely sFlt1-i13, expressed in mice and non-human primates, or sFlt-1e15a, expressed in non-human primates. In healthy pregnant mice, hsiRNA^sFlt1i13 reduced mRNA levels in the placenta by 40%, but did accumulate in maternal liver and kidney [97]. Treatment did not affect the number of pups or the weight of pups [97]. In a baboon uteroplacental ischemia (UPI) PE model, hsiRNA^{sFlt1i13/e15a} reduced serum sFlt-1 levels by 50% 2 weeks post UPI surgery and treatment [97]. Treated animals also had decreased blood pressure and proteinuria. Newborn weights were not statistically significantly different between control groups and PE treated groups [97]. Finally, placenta targeted lipid-polymer nanoparticles carrying sFLT1 siRNA have also been used to decrease placental sFLT1 mRNA levels and circulating sFlt-1 levels in a healthy pregnant mouse model [98]. Maternal liver function as well as fetal weight and number were not changed after nanoparticle treatment [98]. Taken together, these studies demonstrate that engineered nanomedicines could be of substantial use in the treatment of PE.

3.5. Ectopic pregnancies and choriocarcinoma

Certain pregnancy complications, like ectopic pregnancies and placental cancer (choriocarcinoma), are life threatening to the mother and result in limited chance of carrying the fetus to term. Therefore, treatment of these conditions often involves administration of toxic medications to stop embryo or tumor growth or surgical removal [99]. Often these medications can have systemic side effects and surgical procedures have an inherent risk. Nanomedicine could be effective at treating ectopic pregnancies or choriocarcinoma by taking advantage of nanomaterials’ ability to target toxic therapeutics to desired locations, reducing systemic side effects and the need for surgical procedures. However, animal models of ectopic pregnancies and choriocarcinoma are limited due to the inability to induce the conditions reliably. Therefore, most animal models evaluating treatments for these conditions use tumor xenografts to mimic the conditions.

The placenta over-expresses epidermal growth factor receptor (EGFR) [100], making EGFR a potential biomarker to target therapies to the placenta, and therefore choriocarcinoma, in vivo. Researchers targeted bacterially derived nanospheres (EDVs) with EGFR antibodies to JEG-3 tumor xenografts in mice. The EDVs were loaded with doxorubicin (dox) and slowed the growth of the xenografts versus controls, although there was still an increase in tumor volume over 12 days [101]. No biodistribution or safety data was shown. Other studies have used a peptide derived from the malaria parasite that binds placental chrondrotitin sulfate A (plCSA-BP) as a targeting mechanism. Using a JEG-3 xenograft mouse model, it was shown that targeted PLGA/lecithin/PEG nanoparticles accumulated at the tumor site for up to 48 h [102]. When loaded with dox, these targeted nanoparticles significantly reduced xenograft growth and even led to complete reduction of the tumor in some animals, as compared to non-targeted controls. All mice in control groups (PBS, free dox, non-targeted nanoparticles) were dead by day 24, whereas all mice treated with the plCSA-BP targeted nanoparticles survived to day 30, the end of the study [102]. These nanoparticles, when loaded with methotrexate (MTX), have also been used to treat ectopic pregnancy in a mouse model. Using a healthy mouse pregnancy model, plCSA-BP targeted nanoparticles reduced fetal and placental growth resulting in less than a 10% survival rate at gestational day (E)14.5 [103]. These techniques take advantage of placental expressed receptors to target nanoparticles to placenta tissue to treat ectopic pregnancy or choriocarcinoma. Similar nanomedicine strategies may be developed in the future to improve upon the efficacy of the systems reported to date.

4. Fetal conditions that may benefit from nanomedicine

The fetus begins as a quickly developing and communicating mass of cells. There is a large coordination between these cells to develop into a functional system. This process has many steps that can go awry, sometimes to an extreme causing a miscarriage, and sometimes to a lesser extent causing premature birth or birth defects. It is challenging to treat the fetus in utero, but nanomedicine based therapies have begun to be explored in order to overcome disruptions in fetal growth and development. Table 2 summarizes some of the examples in literature to date, which are further elaborated upon in the following sections.

Table 2.

Summary of fetal conditions that have been treated with nanomedicine approaches in preclinical studies.

Disease		Nanomedicine	Therapeutic action	Model	Outcome	Reference
Fetal Growth Retriction		Liposome	Increase IGF-2	Placenta specific Igf-2 knock out mouse	Increased fetal weight and weight distribution	[111]
		Polyplex	Increase Igfl	Uterine artery branch ligation mouse	Labrynth thickness increased and pup weight increased	[113]
		Peptide-miRNA inhibitor complex	Inhibit negative miRNA function	Healthy pregnant mice	Increased placental and fetal weight	[114]
		Liposome	Increase vasodilation via NO donor	eNOS knockout pregnant mouse	Increased fetal weight and spiral artery diameter	[115]
Preterm Birth		Liposome	Reduce prostaglandin and uterine contractions	Healthy pregnant mice	Decreased numbers of preterm births and fetal drug levels	[125,128]
		Nanosuspension	Inhibit prostaglandin function with progesterone	Progesterone antagonist induced PTB mouse	Increased length of pregnancy and number of dams who reached full term	[19]
Fetal Abnormalities	Lafora’s Disease	Liposome	Reprter gene expression	EPM2a null pregnant mouse	Reporter gene expression observed in fetal and neonatal brains	[132]
	β-thalassemia	PLGA nanoparticle	Increase β globin expression	β-thalassemia mouse model	Newborn mice survived to day 500 compared to untreated mice who all died before birth	[136]
	Gaucher Disease	Adeno-associated virus	Increase GCase expression	Neuronopathic Guacher Disease mouse	GCase activity in brain was maintained for up to 18 weeks, mice performed worse at neurological exams	[141]

4.1. Fetal growth restriction (FGR)

One of the most common fetal development complications is fetal growth restriction (FGR), which affects up to 15% of all pregnancies and can influence childhood and adult life [104]. Adults who experienced FGR have a higher lifetime incidence of hypertension, obesity, metabolic syndrome, and diabetes [105]. Maternal disorders, such as hypertension, diabetes, preeclampsia, inflammation, thrombosis, and Lupus, increase the likelihood of FGR. The placentas of FGR babies are approximately 24% lighter than those of healthy babies [104], pointing to a lack of nutrients and oxygen to sustain fetal growth. Although the exact etiology of the condition is not known, there are many potential cellular targets for treatment. It has been shown that available VEGF, PlGF, and insulin-like growth factor-1 (IGF-1) are decreased in FGR [106–108], while soluble sFlt-1 is increased [109]. These physiologic changes can be addressed with therapies using nanomedicine.

Ultimately, FGR is thought to be caused by a dysfunctional placenta and has recently been targeted as a disease potentially treated by nanotherapy. The placenta has also been compared to malignant cells as they have similar characteristics such as high cell proliferation, migratory and invasive properties, and ability to evade the immune system [110]. Therefore, targeting moieties that have been shown to successfully target solid tumors may also target the placenta. Insulin growth factor-2 (IGF-2) is an important growth factor for placental development and growth [111,112] and may be helpful for treating FGR. Using the targeting peptide iRGD (CRGDKGPDC), which has previously been shown to target tumor vessels, liposomes loaded with IGF-2 increased placenta weight in a healthy mouse model and increased fetal weight and fetal weight distribution in a placenta-specific Igf-2 knockout mouse [113]. No change in litter size or number of resorptions was seen after liposome delivery, indicating the treatment is relatively safe [113]. IGF-1, a related growth factor with similar importance in placental development [114], has also been manipulated through gene delivery to treat FGR in mice. Taking advantage of a diblock-copolymer polyplex delivery system and a placenta specific promoter, PLAC1, the IGF-1 gene was directly injected into the placenta of mice with FGR induced by a uterine artery branch ligation [115]. After polyplex injection, the placenta labyrinth thickness increased to baseline indicating that IGF-1 helped placental development in this model [115]. Litter size was unaffected by treatment, but pup weight significantly increased after polyplex treatment [115].

Another peptide known to bind tumor-associated vasculature that has been used to target the placenta is CCGKRK [116]. Peptide-miRNA inhibitor conjugates were synthesized against miRNAs known to be negative regulators of placenta growth. CCGKRK-miRNA inhibitor conjugates were delivered to healthy pregnant mice at three time points during pregnancy. miR-675 inhibitors significantly increased placental weight compared to saline injected controls, while miR-145 inhibitors significantly reduced the variability in placental weight [116]. Both inhibitors increased fetal weight without decreasing mean litter size or increasing number of resorptions [116]. Future studies that build on this work should explore the ability to target the placenta and regulate miRNA expression in models of FGR, rather than in healthy pregnant mice.

In a similar approach, liposomes loaded with a nitric oxide donor (SE175) were targeted to the placenta using CNKGLRNK peptides [117]. Nitric oxide (NO) is produced by endothelial nitric oxide synthase (eNOS) and both are upregulated in FGR, perhaps as a compensatory mechanism for the increased resistance of placental blood flow in FGR [118,119]. It was hypothesized that delivering a NO donor would increase vasodilation in the placenta and reduce resistance. Peptide-targeted liposomes were injected into healthy and eNOS knockout (KO) pregnant mice, as a model of FGR. Treatments did not alter litter size or number of resorptions in healthy mice, but they did increase fetal weight in eNOS KO mice and maintained placental weight more similar to control animals [117]. Treatment also increased spiral artery diameter by 33%, indicating a physiological mechanism for NO [117]. Taken together, this and the aforementioned studies demonstrate there is great potential for engineered nanomedicines to effectively treat FGR.

4.2. Preterm premature rupture of membranes

Preterm premature rupture of membranes (PPROM) is a complication responsible for approximately one-third of preterm births worldwide [120]. PPROM occurs when bacteria enter the amniotic sac either through the vagina or amniocentesis, causing a rupture in the seal and initiating early labor (Fig. 3). The fetus is exposed to bacteria in the uterus and initiates fetal inflammatory response syndrome (FIRS). Approximately 70% of PPROM cases are associated with an amniotic infection, particularly the bacterial presence of Ureaplasma urealyticum, Ureaplasma parvum, and Mycoplasma hominus [120–122]. However, precursors such as behavioral factors (e.g. smoking, poor nutrition), obstetric complications (e.g multiple pregnancies, incompetent cervix), and genetic predisposition can increase the risk of PPROM [120]. Further, PPROM has recently been associated with sterile inflammation, a condition that mimics an inflammatory response despite the lack of microbial presence [120].

Fig. 3.

Potential routes of bacterial infection in pregnant women with preterm premature rupture of membranes (PPROM).

PPROM increases the risk of sepsis for the mother and FIRS for the fetus. To decrease bacterial infection, mothers are often given intravenous or oral antibiotics or steroids. Erythromycin, amoxicillin and clavulanic acid have been tested alone and in combination in previous clinical trials, but these treatment strategies have shown conflicting data regarding fetal risk of cerebral palsy [120,123]. New strategies that can safely treat bacterial infections without increasing fetal risks are needed.

No studies have examined the use of nanoparticle-based therapies to treat PPROM, but one study evaluated the use of azithromycin (AZ)-based treatments in pregnancy. In this study, pregnant sheep were administered a single intra-amniotic (i.a.) dose of AZ or repeated intravenous (i.v.) AZ doses every 12 h from the 80th day of gestation (of 150 days) [121]. The goal of this study was to determine which delivery route would decrease undesirable fetal effects if administered at the 2nd trimester. Sheep were euthanized 120 h after the single i.a. dose or the initial i.v. dose and evaluated for drug distribution, fetal weight, and developmental effects [121]. The single i.a. dose method yielded significant drug accumulation in the amniotic fluid and fetal lungs, but the maternal i.v. method showed drug accumulation in the fetal lung, liver and plasma. Both methods displayed no distinct toxic responses or exposure to the fetus, although the maternal i.v. dose method was favorable for accumulation and ease [121]. The results of this study show promise for using targeted nanoparticles loaded with antibiotics in vivo without disrupting fetal development. A PLGA based nanoparticle coated with PEG is a potential nanomedicine system to be delivered vaginally, as local delivery would grant quick access to the ruptured membrane. Factors such as gestational age, cause of rupture, and severity of bacterial infection should be considered in future research.

4.3. Preterm birth

Preterm birth (PTB) is the most frequent pregnancy complication, affecting more than 400,000 infants in the United States annually [124]. The underlying causes of PTB include spontaneous preterm labor (~45%), PPROM (~30%), multiple pregnancies, cervical incompetence, maternal or fetal infections and other pregnancy conditions [125]. Several other risk factors include short cervical length, multiple births, and genetic predisposition [126]. PTB is traditionally defined as a birth prior to 37 weeks gestation, but extremely preterm infants are born at less than 28 weeks [124]. Infants born extremely preterm face a decreased risk of survival as fetal lung and cardiac development is not complete. PTB is difficult to treat due to the varied underlying causes and pregnancy complications associated. However, several studies in literature have delayed the onset of labor by manipulating hormone levels in pregnant mothers.

One example of the use of engineered drug delivery vehicles to treat PTB was a study that used liposomes encapsulating indomethacin and decorated with oxytocin receptor antagonists (LIP-IND-ORA) [127]. Indomethacin, a non-steroidal inflammatory drug, reduces prostaglandin, which induces labor, production by the uterus. However, indomethacin causes premature closure of the ductus arteriosis [128], may increase intraventricular hemorrhage [129], and is used sparingly due to these fetal toxicity issues. Therefore, encapsulating indomethacin within a targeted delivery system could enable it to be used more freely to treat PTB. ORA was used as a targeting agent to the uterus as oxytocin receptor is over expressed in the pregnant uterus, and it is FDA approved to reduce uterine contractions during preterm labor. Healthy pregnant mice received daily i.v. injections of LIP-IND-ORA beginning on E15 and were then evaluated for liposome distribution on impact on PTB. The results of this study indicated that LIP-IND-ORA displayed 3× higher accumulation in the uterus, liver, placenta and fetus compared to freely delivered ORA [127]. Importantly, the LIP-IND-ORA system reduced drug levels in the fetus following delivery, prolonged pregnancy by 31%, and decreased PTB cases (delivery on or before E19) by 15% [127]. LIP-IND, LIP-ORA, and LIP-IND-ORA all showed statistically similar inhibition of murine uterine contraction indicating that ORA and IND don’t have a synergistic effect [127]. Another study confirmed the effectiveness of targeting via ORA conjugated drug loaded liposomes by comparing ORA-targeted liposomes to non-targeted liposomes [130]. ORA-targeted liposomes loaded with indomethacin were more effective in reducing PTB rates when compared to untargeted liposomes [130]. However, unloaded ORA-liposomes had no effects in uterine contractility tests, unlike the previous study. These results agree with ORA clinical usage review which found that ORA did not increase pregnancy prolongation or neonatal outcomes when compared to placebo [131].

Another strategy that researchers have explored to decrease PTB in pregnant mice is the vaginal administration of a progesterone nanosuspension (NS) [19]. In this study, the progesterone antagonist RU486 was subcutaneously administered to pregnant mice daily beginning on E15 to induce PTB. Separate groups of mice received either RU486 alone, RU486 with daily vaginal administration of a progesterone gel (Crinone®), or RU486 with daily vaginal doses of the progesterone NS [19]. Biodistribution studies indicated that the NS was retained in the uterus and cervix up to 6 h post-administration. Further, oxytocin receptor levels in the cervix were reduced in RU486-exposed mice treated with NS or with the progesterone gel [19]. Excitingly, the percentage of RU486-exposed dams that reached full term to deliver live pups was 55% in the group treated with NS, compared to 32% in the group treated with the gel formulation [19]. Likewise, the median day of parturition was 19.5 in the NS group compared to 16 in the gel group. These results indicate that NS-mediated delivery of progesterone is more effective than gel-mediated delivery at preventing PTB. This finding, combined with those of the above studies, suggest that engineered drug delivery vehicles have great promise as a method to prevent PTB. However, future research should work to refine these treatments and evaluate their use in more advanced models of PTB.

4.4. Fetal abnormalities

Recent progress in the field of gene therapy has opened up the exciting possibility to treat fetal genetic mutations in utero. Here we briefly introduce the topic by discussing advances in the nanomedicine-based treatment of three specific diseases that have been successfully targeted using nanomedicine based therapeutics.

Lafora’s Disease is characterized by epilepsy and cognitive decline due to accumulation of glycogen in the brain, called Lafora bodies [132]. Around 60% of cases are due to a mutation in the EPM2A gene, which encodes a protein that interacts with glycogen [133]. There are currently no treatments for Lafora’s Disease and fatality normally occurs by 10 years after adolescent onset of symptoms. The ability to deliver a functional EPM2A gene to the fetal brain in utero would greatly impact people affected by the disease. While researchers have not yet delivered therapeutic EPM2A genes to fetuses in utero, they have demonstrated the ability to deliver reporter genes to the fetal brain using PEGylated liposomes targeted with transferrin receptor antibodies [134]. Transferrin receptor is over expressed on both the placental surface and the fetal blood brain barrier, making it an optimal target for fetal brain localization [135,136]. Reporter gene expression was found in the fetal and neonatal brains of EPM2a null pups treated with transferrin receptor targeted liposomes, indicating this system has potential to deliver functional genes to the fetal brain [134]. While many more studies are needed to develop a successful treatment for Lafora’s Disease, this study lays the foundation for future implementation of fetal brain gene delivery.

β-thalassemia is a genetic condition in which β globin, a protein important for oxygen transport in red blood cells, is deficient [137]. Anemia is the most common symptom of β-thalassemia but the disease can be deadly if not treated with regular blood transfusions [137]. In order to treat β-thalassemia in mice, peptide nucleic acids (PNAs) and donor DNAs were complexed together into PLGA nanoparticles and delivered to the fetus in utero by either direct venous injection or into the amniotic fluid [138]. In healthy mice, venous delivery of fluorophore-loaded PLGA NPs resulted in high fetal liver (the location of hematopoietic stem cell expansion) accumulation at both E15.5 and E16.5, while amniotic delivery led to accumulation in the fetal lung and gut at E16.5 and later as was anticipated due the onset of fetal breathing and swallowing. A mouse model for β-thalassemia [139] in which no homozygous mice survive postnatally and heterozygous mice survive with anemia was used to test PNA/DNA gene delivery. Fetuses treated in utero with PNA/DNA NPs via venous injection had a higher concentration of hemoglobin and rate of survival into adulthood (500 days) compared to untreated fetuses [138]. This study showed clinically relevant delivery options for treatment of a fetal genetic disorder and lays the groundwork for further in utero treatment options.

Gaucher disease is another fetal abnormality that might benefit from nanomedicine-based fetal gene therapy. Gaucher disease is characterized by organ dysfunction, often of the liver, spleen, bone marrow, and central nervous system, due to the buildup of glucocerebroside lipids [140]. Accumulation is due to a mutation in the gene (GBA) that encodes an enzyme that breaks down glucocerebroside, glucocerebrosidase (GCase) [141]. Depending of the type and severity of the disease, life expectancy can be limited to 2 years. An excellent mouse model of neuronopathic Gaucher disease (nGD) exclusively expresses GCase in the skin resulting in neurodegeneration within 15 days [142]. By intracranially injecting adeno-associated virus (AAV) carrying GBA into fetuses, GCase activity in the brain was maintained at levels similar to wild type mice and treated mice lived for up to 18 weeks, as opposed to 2 weeks untreated [143]. However, treated mice performed worse at various tests, such as Rotarod and grid walk, and weighed less as well [143]. Whether these side effects were due to the AAV system or the intracranial injection method needs to be clarified in further studies. The AAV gene delivery system was also tested in macaques using a green fluorescent protein (GFP) reporter gene. Fetal intracranial injections were performed, resulting in sustained, widespread brain GFP expression [143]. These studies suggest that fetal gene delivery to treat Gaucher disease (and other fetal abnormalities) is a possibility for the future.

5. Future directions

In the last 10 years nanomedicine has increasingly been studied as a way to treat health conditions during pregnancy. While nanotherapeutic approaches have shown promise against several conditions, there are many more diseases that may benefit from intervention with engineered nanomedicines. These conditions and diseases and the potential to use nanomedicine to treat them are discussed below and summarized in Table 3.

Table 3.

Maternal-fetal health conditions and opportunities for future nanomedicine-based therapies.

Condition	Future Opportunities for Research
Bacterial Vaginosis (BV)	Deliver antibiotic-loaded nanoparticles or films to BV mice, evaluate maternal and fetal toxicity of nanoparticles, test different coatings for CVM penetration in BV, evaluate CVM penetration at different stages of menstruation in BV
Diabetes	Preclinically evaluate glucose-responsive nanomaterials to deliver insulin to control hyperglycemina and reduce the chance of hypoglycemia during treatment in pregnancy
Hypertension	Target complement pathway to reduce inflammation, target RAS system to reduce angiotensin II, increase eNOS to induce vasodilation, target endothelin-1 to reduce vasoconstriction
Preeclampsia	Increase VEGF or PlGF, decrease circulation of sFlt-1, increase angiogenesis, manage hypertension, decrease anti-angiogenic factors (AT1-AA), reduce inflammation of placenta, manage altered maternal immune state
Ectopic Pregnancies	Target placenta with peptides or antibodies, deliver toxic drugs to placenta, deliver apoptotic inducing genes to placenta, alter hormone expression to reduce fetal and placental growth
Choriocarcinoma	Target placenta with peptides or antibodies, deliver chemotherapeutic drugs to tumor site
Fetal Growth Restriction (FGR)	Increase VEGF, PlGF, or IGF-1, decrease sFlt-1, decrease placental inflammation, increase placental angiogenesis
Preterm Premature Rupture of Membranes (PPROM)	Study ability of vaginally administered PEG-coated NPs to target rupture sites in pregnant mice, assess effects of gestational age, cause of rupture and severity of bacterial infection
Preterm Birth	Optimize dosing strategy to deliver Indomethacin at earlier gestations or increase drug loading in delivery vehicles
Maternal Cancer	Translate nanomedicines that have been successful against cancer in non-pregnant models to treat cancer in preclinical pregnant models
Mental Illness	Reduce transport of lithium across placenta for treatment of bipolar disorder, deliver SSRIs targeted to mother
Cervical Incompetence	Deliver progesterone vaginally via nanoparticles, stimulate collagen production in cervix
Endometriosis	Induce pseudo pregnancy by targeted delivery or hormones

The ability to precisely deliver engineered nanomedicines in other applications indicates that similar strategies could be used to treat illness during pregnancy. Surface modifications can be used to direct a nanomedicine to a site of interest, or to minimize accumulation in specific sites. For example, strategies to keep nanomedicines in the blood-stream longer, such as PEGylation, could be exploited to limit uptake, and potentially transport, across the placenta. This could allow nanoparticle-based drug delivery vehicles to safely treat conditions in mothers while avoiding harm to the fetus. Stimuli responsive materials that release their cargo in response to an endogenous or exogenous stimulus have also been used in many other applications to minimize off-target effects, and such approaches may be harnessed to improve the treatment of pregnancy complications. For example, many materials disassemble in response to hyperthermia [144,145], and light has also been used as a stimulus to release cargo from photoresponsive carriers [146,147]. Similarly, glucose-responsive insulin delivery systems have shown promise in reducing hypoglycemia and in extending release time based on the concentration of glucose [68–71]. These materials are directly translatable to the maternal-fetal health space and would be excellent systems to treat diabetes in pregnancy. While these stimuli-responsive and other nanosystems have yet to be applied to the maternal-fetal health space, there is a large opportunity for them to be used to treat conditions during pregnancy.

One area where nanomedicine may have impact is the development of novel cancer treatments that are safe for pregnant women to take without harming the developing baby. There are many examples of nanomedicine being used to treat cancer in preclinical studies, but there has been limited cross over to using nanomedicine to treat cancer during pregnancy. Breast cancer is the most common cancer diagnosed during pregnancy, but hematologic malignancies and dermatologic malignancies are diagnosed during pregnancy as well; overall, cancer is diagnosed in about 1 in 2000 pregnant women [148,149]. The number of pregnant women who are diagnosed with cancer is expected to rise as many women are having children later in life. As cancer nanomedicines have been shown to limit off-target effects in non-pregnant animal models, they may also be able to limit off-target effects to the fetus. Thus far, nanoparticle carriers have demonstrated relatively good biocompatibility in pregnant mice [150], but whether nanoparticles carrying toxic anti-cancer agents would also exhibit good biocompatibility for pregnant mothers and fetuses remains to be determined.

There is also a large opportunity for nanomedicine to treat mental illnesses during pregnancy, as around 30% of pregnant women have a psychiatric disorder [151]. While some treatments have been found to be safe during pregnancy, there are still limited options available for pregnant women. For example, bipolar disorder is often managed with the use of lithium, but lithium can freely cross the placental barrier and cause congenital malformations in the fetus, specifically if taken during the first trimester [152]. However, due to the potential severity of a bipolar episode, it may be most beneficial for a pregnant woman to continue her pre-pregnancy therapeutic regimen. Nanomedicines might improve the management of bipolar disorder by preventing lithium from reaching the fetus. With respect to depression, approximately 1 in 10 women are prescribed anti-depressants [153] and many of these women may become pregnant. If a woman has a mild case of depression, it may be reasonable for her to stop taking anti-depressants during pregnancy, but it has been shown that mothers who are depressed may experience low maternal weight gain, preterm birth [154], and increased use of substances like cigarettes and alcohol [155]. Untreated maternal depression is also a strong risk factor for the development of postpartum depression [156]. It has been shown that use of selective serotonin reuptake inhibitors (SSRIs) may increase the risk of neonatal persistent pulmonary hypertension (PPHN) [157]. However, the overall risk of PPHN is low, so increasing the risk will not greatly increase the total number of babies born with PPHN [156]. Overall, the decision whether to continue or discontinue treatment for bipolar disorder and depression during pregnancy depends on a number of factors, and the risks and benefits to both the mother and fetus must be considered. If engineered nanomedicines could be developed to treat these conditions without crossing placental barriers to harm the fetus, it would be transformative for treatment of maternal mental illness during pregnancy.

The ability to keep a drug from crossing the placental barrier using nanomedicine could greatly increase the number of drugs available for pregnant women to take. There are limited examples of nanotherapies being used to treat cervical incompetence (CI) and endometriosis. CI may be hard to treat with nanotherapies as the condition is defined by morphologically disrupted tissues. However, there are some studies that suggest that vaginal delivery of progesterone may extend pregnancy length [158,159]. CI could also potentially be treated by stimulating collagen production and secretion in the cervix. These areas may be an opportunity for the use of nanotherapeutics in the treatment of CI. Interestingly, endometriosis symptoms lessen during pregnancy as the uterine lining is no longer shedding with the menstrual cycle. It has been shown that pseudo pregnancy helps treat endometriosis [160]. In an experimentally induced endometriosis rabbit model, poly(lactic acid) (PLA) nanoparticles loaded with levonorgestrel were able to locally reduce cyst size similar to ovariectomy treatment [161]. This is another opportunity space for nanomedicine in the future.

Besides advancing treatment options for pregnant women, nanomaterials also have potential as diagnostic and imaging tools. Nanostructures can be easily modified with ligands to bind desired biomarkers, many have inherent visualization capabilities, and they can often be separated in straightforward manners for easy quantification. These qualities would be beneficial in a pregnancy context; however, their safety needs to be characterized. Recently a liposomal gadolinium blood-pool contrast agent (Iiposomal-Gd) has been developed to enable safe placental imaging during an MRI in mice [162]. Multiple follow up studies have further provided evidence of a using the liposomal-Gd for placental MRI imaging during pregnancy [163–165]. Other nanosystems have been designed for light based functionality such as the production of heat or acoustic waves after stimulation by specific wavelengths of light [166]. These characteristics allow for accurate, deep, and high-resolution imaging within tissues via photoacoustic endoscopy, focused-scanning photoacoustic microscopy, or photoacoustic tomography. This technology is only recently being applied to imaging during pregnancy, but has the opportunity to be an excellent tool for imaging oxygen levels in the placenta [167]. However, this field is still relatively new and much more research will be needed before being able to be applied clinically.

Engineered nanomedicine has the potential to overcome many treatment barriers surrounding maternal-fetal health. The ability to control many aspects of nanotherapeutics, such as size, shape, material, surface chemistry, and ligands, lends these systems perfectly to the application of maternal-fetal heath. However, application towards this field is still in its infancy leaving many different opportunities available. With further research into engineered nanomedicine for use in maternal-fetal medicine, we will be able to treat pregnant women without the fear of harming the developing baby.

6. Conclusions

Diseases that present during pregnancy, whether chronic or acute, are hard to treat as physiology of the mother changes with pregnancy and there is currently limited knowledge regarding the impact of many drugs on fetal development. There is a need to be able to treat conditions, including pain, mental illness, hypertension, and preterm labor, during pregnancy in a way that is safe for both mother and baby. The recent advancement of engineered nanomedicines makes them intriguing as potential tools to treat diseases and conditions during pregnancy. If nanomedicines can be designed to deliver drugs or genes to specific organs in the mother, placenta, or fetus while reducing off target effects on the non-targeted sites, it would be transformative for maternal-fetal health. Such technologies would dramatically improve the treatment of both pregnancy-induced conditions (e.g., gestational diabetes, preeclampsia, FGR) and pre-existing diseases (e.g., depression, cancer), which currently cannot be treated satisfactorily. Even fatal genetic conditions could potentially be cured through the implementation of fetal gene therapy. While it will take decades for such possibilities to become a clinical reality since engineered nanomedicines for maternal-fetal health are still in their infancy, the potential impact of nanomedicine on outcomes and quality of life for pregnant women and their unborn children is huge. Given this promise, funding agencies and researchers should devote substantial resources and effort towards using nanomedicine to address unmet needs in the treatment of diseases that present during pregnancy.

Abbreviations:

NP

nanoparticle

PEG

poly(ethylene glycol)

CI

Cervical incompetence

FGR

fetal growth restriction

ASD

autism spectrum disorder

BV

bacterial vaginosis

PTB

preterm birth

PPROM

preterm premature rupture of membranes

PLGA

poly(lactic-co-glycolic acid)

PVA

polyvinyl alcohol

HPMC

hydroxypropyl methyl cellulose

GDM

gestational diabetes mellitus

FDA

Food and Drug Administration

ROS

reactive oxygen species

sFlt-1

soluble fms-like tyrosine kinase-1

VEGF

vascular endtothelial growth factor

PE

preeclampsia

ELP

elastin like peptide

PAMAM

poly(amidoamine)

TNF-α

tumor necrosis factor-α

UPI

uteroplacental ischemia

EGFR

epidermal growth factor receptor

dox

doxorubicin

plCSA-BP

placental chrondrotitin sulfate A binding peptide

EDVs

bacterially derived nanospheres

MTX

methotrexate

PIGF

placental growth factor

IGF

insulin-like growth factor

eNOS

endothelial nitric oxide synthase

NO

nitric oxide

FIRS

fetal inflammatory response syndrome

AZ

azithromycin

ORA

oxytocin receptor antagonists

NS

nanosuspension

GCase

glucocerebrosidase

nGD

neuronopathic Gaucher Disease

AAV

adeno-associated virus

GFP

green fluorescent protein

SSRIs

selective serotonin reuptake inhibitors

PPHN

persistent pulmonary hypertension

PLA

poly(lactic acid)

siRNA

small interfering RNA

MOF

metal-organic framework