Therapeutic strategies for colorectal cancer: antitumor efficacy of dopamine D2 receptor antagonists

Abstract

Colorectal cancer (CRC) is one of the leading causes of death, accounting for more than half a million deaths annually. Even worse, an increasing number of cancer cases are diagnosed yearly, and two and a half million new cancer cases are estimated to be diagnosed in 2035. Some antipsychotic drugs, especially those targeting dopamine receptor (DR) D2, demonstrated anticancer activity. Studies have revealed the potential of DRD2 antagonists as anticancer therapeutics, whether alone or as an adjuvant, in treating breast cancer, lung cancer, and others. Emerging evidences indicate DRD2 is involved in the CRC biology, and the association between DRD2 and CRC could be utilized in treating CRC. This study selected DRD2 antagonists with anticancer activity to elucidate the possibility of DRD2 antagonists as new therapeutics for treating CRC.

Introduction

Colorectal cancer (CRC) ranks third in prevalence and second in mortality among cancers globally. More than half a million people die due to CRC annually [1]. The number of cases is increasing, and two and a half million new CRC cases are expected to occur by 2035. Chemotherapy remains one of the key methods in treating patients with CRC for whom surgical treatment is not appropriate or as a postoperative adjuvant to improve the overall condition of patients and limit cancer progression [2]. Either monotherapy based on a single drug, such as 5-fluorouracil, or combination therapy, in which several chemotherapeutics are used simultaneously, could be selected depending on the patient’s condition [3]. However, systemic toxicity of chemotherapy, development of unpredictable resistance, inadequate specificity of cancer, etc., are limitations of current therapy. Therefore, more effective and less toxic anticancer therapeutics remain urgently needed.

The development of new drugs incurs substantial costs that encompass both time and financial resources. The average cost of developing a new drug is more than hundreds of billion United States dollars, and drug repositioning is one of the strategies to overcome this high cost [4]. Drug repositioning, also known as drug repurposing or reprofiling, discovers new therapeutic uses in existing drugs [5]. The time to test the overall toxicity and pharmacokinetics of new molecular entities could be saved, as existing drugs have already been rigorously tested. Moreover, the cost to develop the molecule has already been considered. Thalidomide is a good example of drug repositioning in which an infamous sedative with teratogenicity is transformed into an anticancer medication [6]. Similarly, some targets for cancer therapy have been serendipitously uncovered, initially appearing unrelated or even harmful but later proving to be effective in treatment. These unexpected discoveries have played a crucial role in expanding our understanding of potential targets for cancer treatment, highlighting the intricate and sometimes unpredictable nature of cancer research [7]. A study with prostate cancer cell lines indicated that bromocriptine, an agonist of dopamine receptor (DR) D2 (DRD2), sensitized cancer cells to docetaxel chemotherapy [8], suggesting the repositioning of the drugs working on DRD2.

This review presents the role of dopamine and DRD2 as possible therapeutic targets in cancer treatment. Selected DRD2 antagonists are presented with their anticancer activity, emphasizing the possibility of antipsychotics as new therapeutics in treating CRC.

Dopamine in the colon under physiological and pathophysiological conditions

Dopamine is a neurotransmitter involved in several brain functions, including rewards and motivation [9]. Dopamine exists in the periphery in addition to the central nervous system (Table 1). Dopamine in the periphery is involved in various physiological processes, including gastrointestinal (GI) mobility regulation, hormone release, blood pressure regulation, and salt balance. GI mobility is regulated by several factors in addition to dopamine. The release of norepinephrine (NE) in sympathetic nerves could relax the smooth muscle because NE inhibits the release of acetylcholine (Ach) from motor neurons [10]. Conversely, dopamine is maintained at a relatively higher level in the intestine compared with other sympathetic target organs when the dopamine/NE ratio is concerned [11]. Laboratory experiments indicated that dopamine could relax the jejunum of rats and dogs, and hyperpolarization of guinea pig submucosal neurons was observed.

Table 1.

Concentration of dopamine throughout the periphery

Tissue/organ	Concentration
Central nervous system	1.5 nM~0.1 mM
Carotid body (human infant)	1.3 mM
Lung	0.7 µM
Heart	0.93~3 µM
Plasma	65 pM~35 nM
Spleen (primates)	0.17 µM
Pancreas (primate)	0.22 µM
Stomach	40 nM
Small intestine (rat)	32 nM
Colon	0.14 µM

Data based on [9]

Chron’s disease and ulcerative colitis are known as inflammatory bowel diseases (IBDs). They are considered autoimmune disorders and are characterized by chronic inflammatory conditions in the GI system. A decreased level of dopamine is observed in patients with IBDs and a rodent model of IBDs [9]. Dopamine agonist treatment alleviated the condition of IBDs in a mouse model [12]. The inability to synthesize or store dopamine in enteroendocrine cells and the enteric nervous system is sometimes found in patients with IBDs, indicating that abnormal synthesis and storage of dopamine may cause inflammation [13]. While it remains to be unveiled how the intricate mechanisms of dopamine signaling work to affect the IBDs, one explanation is the role of dopamine in the vascular permeabilization. Dopamine receptor signaling inhibits the vascular permeabilization and angiogenesis which occur in the progress of pathologic IBDs [14].

DRD2 and cancer

Five different types of DRs exist to relay the physiological function of dopamine: DRD1 through DRD5. DRD1 and DRD5 belong to the D1-like DR, and DRD2 through DRD4 belong to the D2-like DR (Fig. 1). They are classified as G protein-coupled receptors (GPCRs). The D1-like DR is coupled to Gα_s, which stimulates the synthesis of cAMP to activate protein kinase A (PKA) signaling, whereas the D2-like DR is coupled with Gα_i, which inhibits the synthesis of cAMP, thereby deactivating PKA signaling. In addition, and the phosphorylation of DRD2 by GPCR-kinase results in the modulation of β-arrestin signaling [15] which suppresses the proliferation of the cells [16]. The modulation of β-arrestin signaling by DRD2 becomes more complicated and results in the activation of ERK, a mitogen-activated protein kinase (MAPK) [17] and the PI3K/Akt pathways [18], as well as delayed calcium release [19]. While there seem no straightforward targets of DRD2 signaling that have been identified yet, STAT3 signaling appears to be regulated by DRD2 signaling too. Obviously, the dimer formation and transport of STAT3 into nucleus is pivotal in cancer cell survival [20], this process is under regulation of DRD2 signaling [21]. As STAT3 signaling is regulated by ERK [22], it is possible ERK is a mediator of this regulation.

Fig. 1.

Signaling Pathways of dopamine Receptor D2 (DRD2). The classical DRD2 signaling is mediated by the inhibition of adenylyl cyclase and protein kinase A (shaded area). DRD2, once internalized, may interact with β-arrestin to modulate the MAPK and STAT3 signaling pathways

The analysis of cDNA in the GI tract revealed the expression of all five types of DRs in the proximodistal axis of the bowel, and immunocytochemistry analysis was positive in the bowel layers, including the neurons, except for DRD4. The analysis of transcripts in the bowel of all-type DR knockout mice revealed the elevated level of mRNAs encoding tyrosine hydroxylase and dopamine transferase (DAT) in DRD2 knockout mice compared with the wild-type (WT) [11]. Additionally, the knockout mice demonstrated a remarkable GI transit time shortening compared with WT and other types of knockout mice, thereby emphasizing the crucial role of DRD2 in controlling GI transit time and colonic mobility. GI transit time shortening would interrupt the digestion and absorption of nutrients.

Interestingly, the relationship between DRD2 and cancer has long been recognized. Treating patients with schizophrenia with DR antagonists lowered the incidence of cancer [23]. Additionally, patients with Parkinson’s disease are less likely to have cancers [24]. It was later reasoned that dopaminergic drugs treating Parkinson’s disease have anticancer activity. Hence, DRD2 is a target revealed serendipitously.

Numerous studies have indicated the importance of DRD2 in cancer therapy. DRD2 expression elevation in both mRNA and protein levels has been recognized in several cancer types. DRD2 has been determined as either a biomarker of cancer prognosis or a possible target in gastric [25], endometrial [26], breast cancers [27], etc. Moreover, DRD2 upregulation could be connected to cancer stemness [28]. Among various cancers, DRD2 has been notably associated with breast and lung cancers, as demonstrated in the following paragraphs and Table 2.

Table 2.

DRD2 in breast and lung cancers

System	Agents/targets	Results/effect
Breast cancer
MCF-7 cells	bromocriptine (DRD2 agonist)	proliferation ↓	[31]
Xenograft mice	sulpiride (DRD2 antagonist)	anticancer effect	[32]
MDA-MB-231, MCF-7, and SKBR3 cells	Mir-4301 (DRD2 downregulation)	apoptosis	[27]
MDA-MB231 and BT549 cells/Mice	DRD2 expression	apoptosis	[33]
Lung cancer
Human	SNP study	DRD2 and nicotine addiction	[34]
Xenograft mice	bromocriptine	proliferation ↓	[35]
Xenograft mice	DRD2 agonist	Cancer angiogenesis ↓	[36]
NSCLC	DRD2 overexpression	Tumor cell growth ↓	[38]

Breast cancer may develop in the presence of high level of prolactin as described elsewhere [29], and the high level of prolactin could be obtained by antagonizing DRD2: DRD2 in the pituitary gland responds to dopamine antagonists to raise the prolactin secretion [29, 30]. In an in vitro study, bromocriptine, an agonist of DRD2, suppressed the proliferation of MCF-7, a human breast cancer cell line, whereas remoxipride reverted the suppressive effect of bromocriptine [31]. To the contrary, sulpiride, an antagonist of DRD2, enhanced the anticancer effect in a metastatic breast cancer xenograft model [32]. The involvement of DRD2 expression in breast cancer cell was again demonstrated in several breast cancer cells: MiR-4301 expression in the breast cancer cell lines resulted in the downregulation of DRD2 and apoptosis [27]. A recent study based on RNA-seq technology suggested DRD2 as a possible tumor suppression gene, and DRD2 triggered programmed cell death in breast cancer cells [33].

Lung cancer and DRD2 are closely related in different ways. First, variant alleles of DRD2 gene are involved with nicotine addiction and smoking [34]. In addition to the established link between smoking and lung cancer, DRD2 is associated with lung cancer through a different context by modulating the growth of lung cancer cells. The inhibition of small cell lung cancer cells by DRD2 agonist bromocriptine in a tumor xenograft mice model was reported in 1994 [35]. Moreover, DRD2 agonists could inhibit lung tumor progress by reducing angiogenesis [36], which is somewhat similar to the role of dopamine in IBDs as described above [14]. In non-small cell lung cancer cells, DRD2 expression is lower than in normal lung tissues, and the overexpression of DRD2 could inhibit the growth of tumor cell [37]. The inhibition of cell proliferation is mediated through the inhibition of NF-κB signaling pathway. While DRD2 co-localizes with CD133, a stem cell marker, in non-small cell lung cancer patient tissues, it appears to remain in an inactive state: the activation of DRD2 by DRD2 agonist results in the inhibition of NSCLC cells [38]. The results from the aforementioned studies in breast and lung cancers appear confusing, as DRD2 signaling sometimes acts as a cancer promoter and other times as a tumor suppressor. These studies illustrate the complex nature of cancer biology related to DRD2.

DRD2 and CRC

There is growing evidence suggesting an association between DRD2 and CRC (Table 3). Early study revealed that metoclopramide, a DRD2 antagonist, could exert an antiproliferative activity to mouse colon tumor cell [39]. Similarly, haloperidol inhibited the growth of rat colon cancer cells [40]. It was shown that dopamine content and dopamine receptor expression are altered in human colon tissue study [41]. Later on, a single nucleotide polymorphism (SNP) study indicated the association of CRC risk and 3 SNPs in DRD2 [42]. In addition, the expression of L-DOPA decarboxylase, an enzyme converting L-DOPA into dopamine, was associated with CRC and considered as a biomarker [43]. One of the proteins relaying the DRD2 signaling is dopamine and cAMP-regulated neuronal phosphoprotein 32 (DARPP-32), which could serve as a marker for worse prognosis in CRC [44]. Recently, it was reported that DARPP-32 promotes CRC cell growth by activating PI3K/AKT signaling [45].

Table 3.

DRD2 in CRC

System	Agents/targets	Results/effect
Mouse xenograft model	Metoclopramide	Antiproliferation	[39]
Rat colon cancer model	Haloperidol	Cancer cell growth↓	[40]
Human colon tissue	Dopamine receptor	Receptor and CRC linked	[41]
Human	SNP study	DRD2 and CRC linked	[42]
Human colon tissue	L-DOPA decarboxylase	Cancer biomarker	[43]
Human colon tissue	DARPP-32	Cancer biomarker	[44]
Human colon tissue	DARPP-32 functional study	DARPP-32↑PI3K/Akt, growth↑	[45]
CRC cell lines (HCT116, LoVo, HCT15, and HT29)	Chlorpromazine	Apoptosis	[46]
HCT116 cells	Thioridazine	Cancer stem cell autophagy	[47]
HT29, HCT116, RKO, LoVo, CT26, and SW480 cells/Mice	Thioridazine	CRC cell autophagy	[48]
HCT116 cells	Domperidone	ERK/STAT3↓ cell death	[49]
Human/animal	Vanoxerine	CRC stem cell↓	[50]

Several DRD2 antagonists have been reported to inhibit the growth of CRC cells. In vitro experiments with human CRC cells showed that chlorpromazine induced the apoptosis of CRC cells in a p53-dependent manner [46]. Similarly, Thioridazine induced the apoptosis of cancer stem cells isolated from human CRC cells HCT116 [47]. Recently, it was shown that thioridazine induced autophagy in human CRC cells [48]. In addition, domperidone exerted its antitumor activity by inhibiting ERK/STAT3 signaling in human CRC cells HCT116 [49]. DRD2, as described above, plays an important role in the growth of colorectal cancers, and DRD2 antagonists have been shown to be effective anticancer therapeutics in treating CRCs. In addition, vanoxerine, while not DRD2 antagonist, seems to suppress CRC stem cell by inhibiting dopamine transporter [50].

DRD2 antagonists as anticancer therapeutics

So far, more than a couple of DRD2 antagonists have been recognized to exert anticancer activity, as presented in the following paragraphs and Table 4.

Table 4.

Cellular Effects of DRD2 antagonists

Compound	Cancer cells	Effects	Reference
Thioridazine	Cancer stem cells from HCT116	Antitumor effect	[47]
Thioridazine	Cervical carcinoma SiHa	Cell death	[53]
Thioridazine	Triple-negative breast cancer cells	STAT3 inhibition	[21]
Thioridazine	Cervical cancer	Akt, PI3K inhibition	[54]
Thioridazine	Leukemia cell	mitochondrial Apoptotic pathway	[55]
Haloperidol	Glioblastoma	ERK signaling↑	[58]
Domperidone	Triple-negative breast Cancer cells	ROS, JAK/STAT3 Signaling↑	[60]
Domperidone	Small cell lung cancer cells	Cell growth↑	[35]

Thioridazine (THD)

THD belongs to phenothiazines, and it is a DRD2 antagonist. THD is used to treat a wide range of psychotic disorders, such as schizophrenia and psychosis [51]. The anticancer activity of THD has been tested numerously, including in vitro and in vivo tests with cancer cells of the ovary, breast, lung, cervix, blood cells, stomach, testicle, liver, head and neck, kidney, prostate, etc. [52]. THD exhibited antitumor effects in CRC stem cells isolated from HCT116 cells [47] and induced cell death in the human uterine cervical carcinoma cell line SiHa [53].

THD suppressed STAT3 to inhibit the self-renewal of breast cancer cells in a DRD2-dependent manner [21]. THD induced cell cycle arrest at the G1 phase, and survival Akt and PI3K pathways were downregulated [54]. Additionally, the induction of apoptosis via caspase activation or the mitochondrial pathway was involved [55].

Haloperidol

Haloperidol belongs to the butyrophenones class and is used as an antipsychotic drug for treating various conditions, including acute psychosis, schizophrenia, bipolar disease, Tourette syndrome, delirium, agitation, and alcohol withdrawal-related hallucinations [56]. It is a DRD2 antagonist and its antipsychotic action results from blocking DRD2 in the mesolimbic dopaminergic pathway. Additionally, haloperidol has antimuscarinic, antiadrenergic, and antihistaminic activities. The antineoplastic, antifungal, and antiviral properties of haloperidol have been explored [57]. An experiment with glioblastoma revealed that haloperidol enhanced the sensitivity of glioblastoma cells to the anticancer drug temozolomide by improving the ERK signaling pathway [58].

Domperidone

Domperidone is a well-known antiemetic that relieves nausea and vomiting. It is a derivative of benzimidazole and a potent DRD2 antagonist [59]. It does not cross the blood–brain barrier and it mainly works in peripheral tissues. It is sold in many countries as an over-the-counter drug, illustrating its safety. Interestingly, a handful of reports indicate that domperidone exerts anticancer activity. Domperidone prevented hepatocellular carcinoma in a mouse model, possibly by elevating prolactin levels. Domperidone increases prolactin levels by interfering with the function of dopamine [59]. An in vitro model may exclude the effect of prolactin, and domperidone elicited anticancer activity in triple-negative breast cancer cells, BT-549 and CAL-51, by modulating reactive oxygen species and JAK/STAT3 signaling [60]. Many studies indicate that DRD2 could be a target for treating cancer, but it should be approached cautiously, as there is a case in which domperidone stimulated the growth of human small cell lung cancer cells by inhibiting bromocriptine, which is a DRD2 agonist [35].

Conclusion

Accumulating evidence, although not definitive, indicates that DRD2 antagonists may function as anticancer therapeutics. Several studies revealed a correlation between DRD2 and cancer. Even though debates remain regarding whether DRD2 agonists or antagonists are suitable choices in cancer treatment [61], DRD2 deserves more attention in understanding cancer therapy, especially in treating CRC.

Intriguingly, seemingly similar cell signaling pathways are modulated by different DRs. One of the disadvantages of drugs working on DRD2 is their selectivity. The cytotoxic concentrations observed in triple-negative breast cancer cells are quite high [60], and one of the DRD2 antagonists, ONC201, works in cells lacking the DRD2 receptor [62]. These drugs may work off-target at such high concentrations. Conversely, domperidone has been in clinical use for a long time, and high doses of domperidone could certainly be tolerated. Domperidone has long been used to regulate bowel movements and may also be effective in treating CRC.

In conclusion, we summarized the possibility of using DRD2 as a target for CRC treatment. Further study will enlighten our understanding of CRC treatment and DRD2.

Funding

This study was funded by Daegu Catholic University research grants in the year 2024.

Declarations

Conflict of interest

The authors claim no conflicts of interest.