Targeted therapy for osteosarcoma: a review

Abstract

Background

Osteosarcoma is a common primary malignant tumour of the bone that usually occurs in children and adolescents. It is characterised by difficult treatment, recurrence and metastasis, and poor prognosis. Currently, the treatment of osteosarcoma is mainly based on surgery and auxiliary chemotherapy. However, for recurrent and some primary osteosarcoma cases, owing to the rapid progression of disease and chemotherapy resistance, the effects of chemotherapy are poor. With the rapid development of tumour-targeted therapy, molecular-targeted therapy for osteosarcoma has shown promise.

Purpose

In this paper, we review the molecular mechanisms, related targets, and clinical applications of targeted osteosarcoma therapy. In doing this, we provide a summary of recent literature on the characteristics of targeted osteosarcoma therapy, the advantages of its clinical application, and development of targeted therapy in future. We aim to provide new insights into the treatment of osteosarcoma.

Conclusion

Targeted therapy shows potential in the treatment of osteosarcoma and may offer an important means of precise and personalised treatment in the future, but drug resistance and adverse effects may limit its application.

Introduction

Osteosarcoma is a common primary bone malignancy in children and adolescents (Belayneh et al. 2021). Currently, the primary treatment for osteosarcoma is surgery combined with auxiliary multi-drug chemotherapy, after which the 5-year survival rate of patients is approximately 77% (Cole et al. 2022; Jiang et al. 2021). However, chemotherapy is not ideal for patients with metastatic or recurrent osteosarcoma (Alexander et al. 2021). To address the poor effects of chemotherapy, researchers have focussed on adjusting the doses, timeframes, and regimens of chemotherapy drugs; however, clinical data have shown that the survival rates of patients have not significantly improved. High-dose chemotherapy can lead to serious side effects, such as neutropenia and infection (Chen et al. 2021). It is, therefore, vital that more effective osteosarcoma treatments are developed. In recent years, targeted therapy, which inhibits tumour growth at the cellular and molecular levels by affecting known gene loci, has been widely used in the treatment of osteosarcoma. This has the advantages of conferring specific anti-tumour activity and low toxicity (Shaikh et al. 2016). This article reviews recent research progress regarding the molecular mechanisms, signalling pathways, therapeutic targets, and potential drugs for targeted osteosarcoma therapy.

Pathogenic molecular mechanisms of osteosarcoma

Osteosarcoma cells are characterised by complex karyotypes, extensive and unstable mutations, and complex interactions between proteins and signalling pathways (Czarnecka et al. 2020); thus, their pathogenic mechanisms have not yet been fully clarified. It is generally believed that the occurrence and development of osteosarcoma may be related to changes in tumour gene copy number and gene rearrangement. Changes in genes mainly affect tumour proliferation and apoptosis by regulating the cell cycle, tumour angiogenesis, and DNA damage repair (Fig. 1). The mammalian target of rapamycin (mTOR) can inhibit tumour growth by regulating the tumour cell cycle (Hua et al. 2019), vascular endothelial growth factor (VEGF) can regulate the activity of tumour cells by influencing the formation of blood vessels around the tumour (Wintheiser and Silberstein 2022; Saraon et al. 2021), and breast cancer (BRCA) genes can mediate DNA damage repair and affect the progression of osteosarcoma (Kovac et al. 2015; Mason-Osann et al. 2018). In addition, epidermal growth factor receptor (EGFR) may affect tumour cells by mediating specific pathways (Sabbah et al. 2020). Current research regarding targeted osteosarcoma therapy is broadly focussed on these targets (Table 1).

Fig. 1.

Overview of targeted osteosarcoma therapies

Table 1.

List of signalling pathways and therapeutic targets for targeted osteosarcoma therapy

Signalling pathway/target	Class	Mechanism	Agents	NCT number	References
PI3K/AKT/mTOR signalling pathway	mTOR inhibitors	Block cell cycle G1 phase, inhibit tumour proliferation	Everolimus, Ridaforolimus, Temsirolimus, Rhaponticin, Piceatannol	NCT01804374, NCT00112372, NCT01614795	(Hua et al. 2019; Yu et al. 2021; Hu et al. 2016; Pignochino et al. 2013; Grignani et al. 2015; Chawla et al. 2012; Fleuren et al. 2014; Wagner et al. 2015; Mickymaray et al. 2021; Wang and Li 2020)
Cyclin-dependent kinases (CDKs)	CDK4/6 inhibitors	Block cell cycle G1 phase, inhibit tumour proliferation	Palbociclib, Abemaciclib, Flavopiridol	NCT04040205	(Zhou et al. 2016, 2018; Lukasik et al. 2021; Oshiro et al. 2021; Rader et al. 2013; Goetz et al. 2017; Wang and Bao 2022; Zocchi et al. 2018)
	CDK2 inhibitors, WEE1 inhibitors	Affect cell cycle G1/S and G2/M, inhibit tumour proliferation	SCH 727965, AZD1775	–	(Lockwood et al. 2011; Wei et al. 2020; Sayles et al. 2019; Fu et al. 2011; Yang et al. 2015; Matheson et al. 2016; Moiseeva et al. 2019; Takebe et al. 2021)
Aurora kinase	Aurora inhibitors	Inhibit cell mitosis, reduce risks of cancer	VX-680, ZM447439, MLN8237	NCT01154816	(Tavanti et al. 2013; Song et al. 2020; Wu et al. 2020; Zhu et al. 2014; Niu et al. 2015; Mosse et al. 2019)
MDM2	MDM2 inhibitors	Inhibit MDM2 ubiquitin-linking function and block interaction with p53	SAR405838	–	(Suehara et al. 2019; Sciot 2021; Synoradzki et al. 2021; Wang et al. 2014)
TP53 mutation	WEE1 inhibitors	Inhibit WEE1, which in turn replaces the regulatory role of TP53, initiate DNA repair	AZD1775, ZN-c3	NCT04833582	(Synoradzki et al. 2021; Tang et al. 2019; Chen et al. 2014; Puzik et al. 2021; Seligmann et al. 2021; Cole et al. 2020; Meng et al. 2021; Huang et al. 2021)
MYC	MYC inhibitors	Inhibit the transformation of bone marrow mesenchymal stem cells into osteosarcoma-like cells	10058-F4, Omomyc	–	(Czarnecka et al. 2020; Feng et al. 2020; Wahlstrom and Henriksson 2015; Han et al. 2012; Chen et al. 2018; Wang et al. 2017; Sorolla et al. 2020; Madden et al. 2021)
IGF-R	IGF-R/IGF-1R inhibitors	Block the interaction of IGF-1R with ligands, mediate the PI3K/AKT pathway to inhibit proliferation	Cixutumumab, Picropodophyllin, PQ401	NCT00831844	(Martin et al. 2021; Tzanakakis et al. 2021; Ameline et al. 2021; Cao et al. 2020; Weigel et al. 2014; Duan et al. 2009; Qi et al. 2019)
VEGFR	VEGFR inhibitors	Block VEGF binding to VEGFR, inhibit tumour angiogenesis	Apatinib, Sorafenib, Cabozantinib, Regorafenib, Pazopanib, Bevacizumab	EudraCT 2007-004396-19, NCT02243605, NCT02389244, NCT02048371	(Assi et al. 2021; Parmar and Apte 2021; Mohamed et al. 2020; Long et al. 2021; Liu et al. 2017, 2021; Xie et al. 2021; Man et al. 2021; He et al. 2021; Mei et al. 2014; Grignani et al. 2012; Italiano et al. 2020; Higuchi et al. 2021; Duffaud et al. 2019; Davis et al. 2019; Elete et al. 2020; Frankel et al. 2022; Navid et al. 2017)
PDGFR	PDGFR inhibitors	Inhibit tumour angiogenesis and affect the tumour microenvironment via paracrine	STI-571, Olaratumab	NCT00031915	(Xu et al. 2018; Andrae et al. 2008; Oda et al. 1995; Zvi et al. 2021; Xing et al. 2020; Morita and Sasaki 2021; Yamaguchi et al. 2015; Lowery et al. 2018; Higuchi et al. 2019)
BRCA	PARP inhibitors	Inhibit PARP to block DNA repair in BRCA-mutated tumours	Olaparib	NCT04417062	(Kovac et al. 2015; Rosen and Pishvaian 2014; Park et al. 2018)
ATRX	EZH2 inhibitors, WEE1 inhibitors	Inhibit ATRX to activate the telomere alternative elongation (ALT) pathway	Tazemetostat, MK1775, PD0166285	–	(Mason-Osann et al. 2018; Rickel et al. 2017; Ji et al. 2017; Masliah-Planchon et al. 2018; Liang et al. 2020; Qadeer et al. 2019; Nigris et al. 2021; Kreahling et al. 2013; PosthumaDeBoer et al. 2011)
EGFR	EGFR inhibitors	Mediate Ras/ERK and PI3K/AKT pathways to inhibit tumour growth	Trastuzumab, Cetuximab, OST31-164	NCT00023998, NCT04616560, NCT04974008	(Sabbah et al. 2020; Marchio et al. 2021; Tumbrink et al. 2021; Nakamura et al. 2021; Sheng et al. 2017; Wan et al. 2019; Sevelda et al. 2015; Ebb et al. 2012; Liu et al. 2015; Pahl et al. 2012; Newswire 2021a, 2021b)

Targeted therapy by modulating the cell cycle

Mammalian target of rapamycin pathway

mTOR is an important serine/threonine kinase. As a downstream regulator of PI3K, mTOR is involved in the PI3K/AKT signalling pathway and is responsible for regulating protein production, the cell cycle, and cell survival (Hua et al. 2019). A study found that mTOR signalling is abnormally activated in osteosarcoma cells, which is one of the crucial mechanisms of the occurrence and development of osteosarcoma (Yu et al. 2021). Hu et al. (2016) found that patients with osteosarcoma with higher mTOR expression had poorer prognosis, suggesting that mTOR is a potential oncological therapeutic target.

Several mTOR inhibitors (everolimus, ridaforolimus, and temsirolimus) are currently approved for clinical cancer treatment. As a rapamycin derivative, everolimus inhibits the proliferation of osteosarcoma cells by blocking the G1 phase of the cell cycle. Pignochino et al. (2013) demonstrated in preclinical models that everolimus in combination with sorafenib improved everolimus anti-tumour efficacy. However, a phase II clinical trial (NCT01804374) showed that the combination was not so effective in patients with advanced or unresectable osteosarcoma, which has also limited further drug development of everolimus (Grignani et al. 2015). Another phase II trial (NCT00112372) demonstrated that ridaforolimus had clinical effectiveness in patients with advanced osteosarcoma (Chawla et al. 2012). Temsirolimus, similar to everolimus, has exhibited anti-tumour activity in combination with cisplatin or bevacizumab in in vitro assays (Fleuren et al. 2014); however, a phase II clinical trial (NCT01614795) has shown unsatisfactory results (Wagner et al. 2015). In addition, it has been reported that treatment with both rhaponticin and piceatannol can cause the apoptosis of osteosarcoma cells by inhibiting the PI3K/AKT/mTOR signalling pathway (Mickymaray et al. 2021; Wang and Li 2020); this is expected to be a potential drug target for osteosarcoma in future.

Cyclin-dependent kinases

Cyclin-dependent kinases (CDKs) are a class of serine/threonine protein kinases that regulate the cell cycle (Zhou et al. 2016). There are currently 20 CDK isoforms known to mediate the regulation of the cell cycle (Lukasik et al. 2021), of which CDK4/6 may have the potential to treat osteosarcoma (Zhou et al. 2018). The upregulation of CDK4/6 can lead to increased levels of Cyclin-D; the complex formed by Cyclin-D and CDK4/6 can phosphorylate the Rb gene, which in turn releases the transcription factor E2F to affect cell proliferation (Lukasik et al. 2021). Palbociclib was the first CDK4/6 inhibitor to be approved by the Food and Drug Administration (FDA) for breast cancer. Zhou et al. (2018) found that palbociclib can induce osteosarcoma cell cycle arrest and inhibit DNA synthesis in the G1 phase by inhibiting the expression of CDK4/6. Oshiro et al. (2021) used palbociclib in a mouse model of osteosarcoma and found that palbociclib reduced resistance to doxorubicin. Abemaciclib, a CDK4/6 inhibitor that blocks cell cycle transition from G1 phase to S phase by mediating CDK4/6, is approved for use in advanced HR+/HER− breast cancer (Rader et al. 2013; Goetz et al. 2017). Wang and Bao (2022) found that abemaciclib and doxorubicin could have a synergistic effect on osteosarcoma through in vitro experiments. A relevant clinical trial is ongoing (NCT04040205). Furthermore, the broad-spectrum CDK inhibitor flavopiridol has recently been shown to inhibit the metastasis of osteosarcoma cells (Zocchi et al. 2018).

Cell cycle protein E1 (CCNE1) is also frequently involved in the progression of osteosarcoma. Unlike CDK4/6, CCNE1 leads to phosphorylation of pRB1 through interaction with CDK2, which, in turn, affects the cell cycle G1/S transition and promotes tumourigenesis (Lockwood et al. 2011). Wei et al. (2020) found that CCNE1 was highly expressed in osteosarcoma and was correlated with clinicopathology. Therefore, CCNE1 has the potential to be a targeted therapy for osteosarcoma. Due to the interrelationship between CCNE1 and CDK2 in the cell cycle, CDK2 inhibitors are currently considered for targeted therapy in CCNE1-positive tumours (Sayles et al. 2019). Fu et al. (2011) found that the CDK2 inhibitor SCH727965 (dinaciclib) had an inhibitory effect on osteosarcoma cells through in vitro experiments. Other CDK2 inhibitors, such as SNS-032, have also been shown to play a role in CCNE1-positive ovarian cancer (Yang et al. 2015). In addition, WEE1 inhibitors have been proposed to affect the expression of CCNE1, which leads to premature entry into the S-phase by blocking Y15 phosphorylation in CDK1 and CDK2, thereby exacerbating the cell cycle dysregulation of G1/S and G2/M in the cell cycle, leading to apoptosis (Matheson et al. 2016; Moiseeva et al. 2019). Results of a phase I clinical trial (NCT01748825) suggest that the WEE1 inhibitor AZD1775 may be effective in patients with CCNE1-overexpressing solid tumours (Takebe et al. 2021). However, the idea that “WEE1 inhibitors can target CCNE1-positive osteosarcoma” still needs to be supported by more experimental data (Fig. 2).

Fig. 2.

Potential targeted osteosarcoma therapies based on cell cycle modulation

Aurora kinase

Aurora kinase is a serine/threonine kinase responsible for regulating mitosis and is divided into three subtypes: Aurora A, Aurora B, and Aurora C. Alterations in its own activity can lead to abnormal mitosis, which in turn affects the tumour cell cycle. At present, Aurora A and B have already been widely studied in the context of osteosarcoma (Tavanti et al. 2013). Aurora B can promote the growth and metastasis of osteosarcoma cells by activating the NPM1/ERK/NF-κb/MMP pathway (Song et al. 2020). Wu et al. (2020) believed that knocking out Aurora B could reduce the expression of the mTOR/ULK1 pathway, thereby increasing autophagy and inhibiting the metastasis of osteosarcoma cells. Some studies have also shown that Aurora A has potential as a targeted osteosarcoma therapy (Zhu et al. 2014).

Commonly used aurora inhibitors include VX-680, ZM447439, and MLN8237 (alisertib). VX-680 and ZM447439 are broad-spectrum inhibitors of Aurora, which can inhibit all three subtypes. Tavanti et al. (2013) showed that VX-680 and ZM447439 can induce osteosarcoma cell apoptosis by blocking the G2/M phase of osteosarcoma cells. In addition, the combination of VX-680 with traditional osteosarcoma chemotherapy drugs can improve its therapeutic effects. However, combination with ZM447439 has the opposite effects. MLN8237 is a second-generation Aurora inhibitor that selectively blocks the expression of Aurora A, leading to apoptosis. Studies have reported that MLN8237 can activate the mitochondria-dependent apoptosis pathway and inhibit the PI3K/AKT/mTOR pathway, thereby inducing autophagy in osteosarcoma cells (Niu et al. 2015). However, results from an alisertib clinical trial in children with recurrent or refractory solid tumours or leukaemia (NCT01154816) demonstrated that 8 out of 10 patients with osteosarcoma showed progressive disease after treatment with alisertib (Mosse et al. 2019). The result could influence the further development and application of alisertib.

Murine double minute 2

Murine double minute 2 (MDM2), a p53 ubiquitin ligase, mainly regulates the transcriptional activity and stability of p53. Studies have demonstrated that MDM2 is associated with the recurrence or distant metastasis of osteosarcoma (Suehara et al. 2019). Sciot (2021) found that the overexpression of MDM2 is common in low-grade osteosarcoma, such as parosteal osteosarcoma. When the degree of malignancy of osteosarcoma increases, the overexpression of MDM2 does not disappear. Targeted therapy against MDM2 includes inhibition of the ubiquitin-linking function of MDM2 and blocking of the interaction between MDM2 and p53 (Synoradzki et al. 2021). An MDM2-specific inhibitor, SAR405838, has been shown to exhibit anti-tumour activity in SJSA-1 cell line osteosarcoma mouse and wild-type p53 models (Wang et al. 2014). There are currently no clinical trials targeting MDM2 in the treatment of osteosarcoma; therefore, data are still needed to demonstrate the effectiveness of this in osteosarcoma.

TP53 mutation

TP53 is one of the most common tumour suppressor genes and a key regulator of the cellular G1/S phase checkpoint. However, its mutation can lead to the loss of tumour suppressor function, which allows tumour cells to survive, proliferate, and metastasise (Tang et al. 2019). Chen et al. (2014) used genome sequencing technology to find that approximately 90% of patients with osteosarcoma display TP53 gene mutations or structural changes in proteins. In addition, the mutation of TP53 is also an indicator of osteosarcoma in Li–Fraumeni syndrome (Synoradzki et al. 2021; Puzik et al. 2021). Due to its high mutation rate and oncogenic potential, TP53 mutation is considered a promising therapeutic target for osteosarcoma. Currently, there are no drugs that directly target TP53 mutations. As a WEE1 inhibitor, AZD1775 can affect the function of CDK1 in regulating the G2/M phase of cells by inhibiting WEE1, thereby replacing the regulation of TP53 to initiate DNA repair (Seligmann et al. 2021). A phase I clinical trial showed that AZD1775 exhibits anti-tumour activity in the treatment of paediatric osteosarcoma (Cole et al. 2020). Another WEE1 inhibitor, ZN-c3, also has good anti-tumour activity with a higher safety profile than AZD1775 and is suitable for use in combination with other drugs (Meng et al. 2021; Huang et al. 2021). A clinical study of ZN-c3 in combination with gemcitabine in subjects with osteosarcoma is ongoing (NCT04833582). Therefore, more clinical trials are needed to explore the potential therapeutic effects of targeting mutant TP53 in osteosarcoma.

MYC

The MYC gene is an oncogene encoding a nuclear protein and was first discovered in Burkitt lymphoma (Feng et al. 2020). Its family includes C-MYC, N-MYC, and L-MYC; and all three isoforms encode intranuclear DNA-binding proteins related to cell cycle regulation (Wahlstrom and Henriksson 2015). Among these, the C-MYC gene is closely related to osteosarcoma (Czarnecka et al. 2020). Studies have shown that c-MYC can mediate osteosarcoma proliferation through the MEK-ERK pathway (Han et al. 2012). Chen et al. (2018) found that MYC expression is significantly upregulated in metastatic osteosarcoma samples. Furthermore, overexpression of MYC combined with silencing of the retinoblastoma (Rb) gene converts bone marrow–derived mesenchymal stem cells into osteosarcoma-like cells (Wang et al. 2017). Therefore, MYC may influence osteosarcoma by regulating the cell cycle. However, although c-MYC may play a key role in the progression of osteosarcoma, Sorolla et al. (2020) concluded that direct targeting of this gene was not feasible. Madden et al. (2021) suggested that c-MYC is an intrinsic disordered protein (IDP) in nature, and conventional small-molecule targeted drugs did not selectively bind to MYC. Furthermore, due to the location of c-MYC in the nucleus, inhibitors may not be able to penetrate and reach the nucleus. There are still no relevant clinical trials of targeted inhibitors that directly target c-MYC. Feng et al. (2020) first described 10058-F4 as a small-molecule MYC inhibitor that inhibits the proliferation of tumour cells in the KHOS and U2OS osteosarcoma cell lines. In addition, Omomyc inhibition of c-MYC has been studied quite extensively; however, more relevant trial data are still required (Madden et al. 2021).

Insulin-like growth factor receptor

The insulin-like growth factor (IGF) system consists of three ligands: IGF-1, IGF-2, insulin, and three corresponding tyrosine receptors (IGF-1R, IGF-2R, and IR), as well as six different IGF-binding proteins (IGF-BPS1–6). The IGF system is involved in growth and development and is responsible for regulating the autocrine and paracrine functions of hormones in human adulthood (Martin et al. 2021). It has been reported that the IGF-1 receptor (IGF-1R) can promote the early development of osteosarcoma by activating the PI3K/AKT pathway and plays a role in late lung metastasis of osteosarcoma via the RAS/MAPK pathway (Tzanakakis et al. 2021). Ameline et al. (2021) screened 253 cases of osteosarcoma and found that igF-1R expression was significantly associated with upregulation of the MYC gene and PI3K/AKT/mTOR pathway, suggesting that IGF-1R may affect osteosarcoma progression by regulating cell cycle arrest. Another study showed that downregulation of IGF-1R inhibits the proliferation of osteosarcoma cells and promotes apoptosis (Cao et al. 2020). Therefore, inhibition of the IGF system may inhibit these pathways and confer the desired therapeutic effects.

There have been several studies conducted on IGF-R/IGF-1R inhibitors for the treatment of osteosarcoma. Cixutumumab is an anti-IGF-1R monoclonal antibody that binds IGF-1R with high affinity and blocks the interaction between IGF-1R and its ligand to inhibit tumour growth. In a phase II study (NCT00831844), cixutumumab showed good efficacy in treating adolescent patients with osteosarcoma, 9% of whom displayed stable disease, but poor efficacy in treating patients with recurrent osteosarcoma (Weigel et al. 2014). Another study showed that picropodophyllin, an IGF-1R inhibitor, not only inhibited the growth and proliferation of osteosarcoma cells, but also increased the sensitivity of osteosarcoma cells to doxorubicin (Duan et al. 2009). In addition, Qi et al. (2019) reported that the IGF-1R inhibitor PQ401 can inhibit the proliferation and metastasis of osteosarcoma and improve the sensitivity of metastatic osteosarcoma to chemotherapy. However, this conclusion is drawn from in vitro data, and more relevant clinical trials are required to demonstrate the efficacy of PQ401.

Anti-angiogenesis targeted therapy

Vascular endothelial growth factor receptor

VEGF and its receptor (VEGFR) can promote the proliferation and angiogenesis of vascular endothelial cells and participate in the regulation of the tumour cell microenvironment. The VEGF family includes VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, and placental growth factor. VEGFRs can be divided into VEGFR-1, VEGFR-2, and VEGFR-3, among which VEGFR-1 and VEGFR-2 are mainly involved in angiogenesis and are selectively expressed in vascular endothelial cells, while VEGFR-3 is expressed in lymphoendothelial cells and tumour vascular cells involved in lymphangiogenesis (Assi et al. 2021). Activation of VEGF and VEGFR leads to increased vascular permeability and promotion of tumour angiogenesis, thus accelerating tumour progression (Parmar and Apte 2021). Mohamed et al. (2020) found that VEGF overexpression is closely related to poor prognosis in patients with osteosarcoma. VEGF-A expression has been reported to be associated with pulmonary metastasis of osteosarcoma (Assi et al. 2021).

Currently, VEGF-related drugs include apatinib, sorafenib, cabozantinib, regorafenib, pazopanib, and bevacizumab. Apatinib, an inhibitor of VEGFR-2, is widely used in the treatment of advanced soft tissue sarcoma (Long et al. 2021). Apatinib promotes autophagy and apoptosis in osteosarcoma by regulating the VEGFR-2/STATE3/BCL-2 pathway (Liu et al. 2017). In addition, apatinib has been reported to improve the chemotherapy efficacy of ifosfamide combined with etoposide in the treatment of recurrent osteosarcoma (Xie et al. 2021). Clinical trials for the treatment of osteosarcoma using apatinib are currently ongoing. Sorafenib is an oral angiogenesis inhibitor that can target VEGFR1-3 and is widely used in the treatment of liver cancer and renal cell carcinoma (Man et al. 2021; He et al. 2021). It has been reported that sorafenib inhibits the proliferation of osteosarcoma by modulating the VEGFR-2, RET, and MEK/ERK pathways (Mei et al. 2014). In a phase II trial (EudraCT 2007-004396-19), 35 patients with relapsed or unresectable high-grade osteosarcoma treated with a standard dose (400 mg, twice daily) of sorafenib had a PFS of 46% at 4 months, with 3 (8%) partial responses (PRs) and 12 (34%) stable diseases (SDs) (Grignani et al. 2012). A phase II clinical trial of cabozantinib (NCT02243605) showed no progression in 14 out of 42 (33%) patients with advanced osteosarcoma, demonstrating the good anti-tumour activity and tolerability of cabozantinib in advanced osteosarcoma (Italiano et al. 2020). Regorafenib is a multi-target inhibitor that can target VEGFR1-3, RET, BRAF, among others, and has stronger pharmacological effects than sorafenib (Liu et al. 2021). Higuchi et al. (2021) reported that regorafenib showed good efficacy in drug-resistant osteosarcoma mouse models. Two recent phase II trials (NCT02389244 and NCT02048371) have demonstrated the efficacy of regorafenib in the treatment of refractory osteosarcoma (Duffaud et al. 2019; Davis et al. 2019). In addition, there is literature indicating the potential and feasibility of pazopanib and bevacizumab in the treatment of recurrent or metastatic pulmonary osteosarcoma (Elete et al. 2020; Frankel et al. 2022; Navid et al. 2017).

Platelet-derived growth factor receptor

The platelet-derived growth factor (PDGF) family consists of four ligands (PDGF-A, PDGF-B, PDGF-C, and PDGF-D) and two receptors (PDGFR-α and PDGFR-β) (Xu et al. 2018), which are chemically induced and participate in biological processes such as erythropoiesis, bone formation, and angiogenesis (Andrae et al. 2008). PDGF and its associated receptor (PDGFR) can directly induce tumour proliferation through autocrine mechanisms and can also affect tumour stromal cells through paracrine mechanisms, promote tumour angiogenesis, and overcome hypoxia in the tumour microenvironment (Xu et al. 2018). Studies have shown that the expression of PDGF and PDGFR is significantly heterogeneous in osteosarcoma (Oda et al. 1995), and Zvi et al. (2021) suggested that PDGFR-β is associated with poor prognosis in osteosarcoma. PDGFR-β inhibits the invasion and metastasis of osteosarcoma cells by affecting epithelial–mesenchymal transition (Xing et al. 2020).

Imatinib mesylate (STI-571), a tyrosine receptor inhibitor that targets PDGFR signalling, is widely used in the treatment of chronic myeloid leukaemia (CML) (Morita and Sasaki 2021). Yamaguchi et al. (2015) showed that STI-571 combined with cisplatin could produce synergistic anti-proliferative effects in animal models of osteosarcoma with positive PDGF-B expression. However, the results of a phase II clinical trial (NCT00031915) showed that STI-571 was less effective as a single agent in the treatment of osteosarcoma; only 5 of 27 patients with osteosarcoma did not exhibit disease progression. Olaratumab selectively binds to PDGFR-α and disrupts receptor–ligand interactions, thereby affecting tumourigenesis. Similar to STI-571, olaratumab appeared to be more effective in combination with other drugs than on its own. Lowery et al. (2018) showed that olaratumab can delay the progression of paediatric osteosarcoma, but when combined with cisplatin or doxorubicin, can enhance drug activity. In animal models, combination treatment with olaratumab and cisplatin reduced chemotherapy resistance in osteosarcoma (Higuchi et al. 2019). Achieving clinical efficacy with PDGF/PDGFR targeting may, therefore, require combination treatment with other drugs, and more clinical trials are needed to confirm this point in the future.

DNA damage repair targeted therapy

Breast cancer gene and poly ADP-ribose polymerase inhibitors

Poly ADP-ribose polymerase (PARP) is a DNA-repair enzyme. Conventional wisdom holds that PARP inhibition can lead to DNA damage and apoptosis, especially in tumour cells with homologous recombination deficiency (HRD). The BRCA gene is currently known to be associated with HRD (Rosen and Pishvaian 2014); the inhibition of PARP blocks the DNA-repair process in BRCA-mutated tumours, thereby enabling apoptosis via a synergistic lethal pathway. Approximately 80% of the osteosarcoma samples studied by Kovac et al. (2015) displayed the genetic characteristics of BRCA expression; it, therefore, may follow that PARP inhibitors could be effective in treating patients with BRCA-positive osteosarcoma. The most used PARP inhibitor in clinical practice was olaparib. Park et al. (2018) found that olaparib can enhance the chemotherapeutic efficacy of doxorubicin, especially in treating osteosarcoma with positive BRCA1/2 and PARP1 expression. A single arm phase II clinical trial of olaparib in combination with ceralasertib in two cohorts of patients aged 12–30 with recurrent osteosarcoma (NCT04417062) is currently ongoing.

Alpha-thalassaemia mental retardation syndrome X gene and alternative lengthening of telomere pathway

The alpha-thalassaemia mental retardation syndrome X (ATRX) gene is a chromatin remodelling protein that was first identified in ATRX syndrome. It has been reported that knocking down ATRX can activate the alternative lengthening of telomere (ALT) pathway, thus stimulating telomere lengthening and preventing tumour cell senescence (Mason-Osann et al. 2018). The ALT pathway is ubiquitous in osteosarcoma, and the ATRX gene can be repeatedly mutated in osteosarcoma cells as a tumour suppressor gene (Rickel et al. 2017). Ji et al. (2017) first reported two cases of osteosarcoma in children with ATRX syndrome. In addition, Masliah-Planchon et al. (2018) reported two cases of ATRX-positive osteosarcoma. This suggests that targeted inhibition of ATRX may be effective in treating some cases of osteosarcoma. Liang et al. (2020) found that the inhibition of WEE1 resulted in S-phase arrest and DNA damage in ATRX-mutant tumour cells and led to programmed tumour cell death. In addition, Qadeer et al. (2019) found that the inhibition of EZH2 induced upregulation of neuronal genes bound by REST and/or H3K27me3 in ATRX-mutant neuroblastoma, leading to apoptosis. Therefore, WEE1 and EZH2 have the potential to target ATRX-mutant osteosarcoma. Currently, several targeted factor inhibitors, such as EZH2 (tazemetostat) and WEE1 inhibitors (MK1775 and PD0166285), have been shown to be effective against advanced osteosarcoma cells and improve their sensitivity to radiotherapy (Nigris et al. 2021; Kreahling et al. 2013; PosthumaDeBoer et al. 2011).

Epidermal growth factor receptor

EGFR affects the proliferation and signal transduction of epithelial growth factor cells, which are members of the ErbB receptor family, including HER-1, HER-2, HER-3, and HER-4. EGFR binds to corresponding ligands to form homologous dimers, promoting EGFR phosphorylation and thus regulating cell proliferation and apoptosis (Sabbah et al. 2020). Studies have shown that the targeting of EGFR is effective in treating breast (Marchio et al. 2021), lung (Tumbrink et al. 2021), and colorectal cancers (Nakamura et al. 2021) as well as other malignant tumours. However, the application and mechanism of action of EGFR in osteosarcoma remain controversial. Sheng et al. (2017) found that EGFR could mediate the Ras/ERK signalling pathway, inhibit epithelial–mesenchymal transition, and promote the proliferation of osteosarcoma cells. Some researchers have also proposed that changes in the ubiquitination of EGFR can activate the PI3K/AKT pathway and affect osteosarcoma growth (Wan et al. 2019) (Fig. 3). However, Sevelda et al. (2015) found that, while the use of EGFR inhibitors in osteosarcoma cell lines did not reduce proliferative activity, it significantly inhibited their ability to migrate. Based on this, different theories have been proposed regarding the mechanism of action of EGFR, and it is believed that EGFR may not be the main driving force behind the proliferation and growth of osteosarcoma cells. Therefore, the correlation between EGFR and osteosarcoma needs to be further investigated.

Fig. 3.

Potential anti-angiogenesis, DNA damage repair, and epidermal growth factor receptor-targeted osteosarcoma therapies

EGFR-associated inhibitors: (1) trastuzumab is a HER-2-specific monoclonal antibody. The results of a phase II trial (NCT00023998) showed that trastuzumab treatment in HER-2-positive patients resulted in no significant improvement in overall survival compared with that in HER-2-negative patients (Ebb et al. 2012). However, another study (Liu et al. 2015) showed that combination treatment with trastuzumab and zoledronate inhibited the growth of HER-2-positive osteosarcoma cells. Although this was an in vitro trial, it also demonstrated its potential value in treating HER-2-positive osteosarcoma. A phase II clinical trial of trastuzumab in combination with deruxtecan for the treatment of patients with newly diagnosed or recurrent osteosarcoma (NCT04616560) started in 2021; however, it is currently suspended. (2) Cetuximab is a monoclonal antibody that blocks EGFR signalling by competitively inhibiting the binding of EGFR to ligands. Cetuximab has been reported to enhance the activity of NK cells and inhibit the growth of osteosarcoma cells (Pahl et al. 2012). (3) OST31-164, an attenuated live Listeria monocytogenes-expressing a tLLO-HER2 fusion protein, was observed to activate antigen-presenting cells (APCs) of the canine immune system in vitro to produce potent HER2-specific T cells and target HER2-expressing tumour cells (Newswire 2021a). A phase II clinical trial (NCT04974008) is currently planned for the maintenance treatment of OST31-164 in patients with a recent history of recurrent pulmonary osteosarcoma that has been completely resected (Newswire 2021b).

Limitations of targeted therapy

With the progression of medical technology and molecular biology, there has been significant focus on the development of targeted anticancer drugs. Currently, 89 targeted therapy drugs have been approved by the FDA and NMPA (National Medical Products Administration) for the treatment of various malignancies (Zhong et al. 2021), and many additional drugs have entered phase II clinical trials. Although targeted therapy has good potential for application, some problems remain to be solved. Use of almost all targeted drugs eventually leads to the development of drug resistance. Drug resistance is related to many factors, including gene mutations and amplification, and dysregulation of apoptosis and autophagy (Schram et al. 2017; Mele et al. 2020), among which gene mutations are the primary cause (Zhong et al. 2021). In response to this, Pottier et al. (2020) proposed that the combined application of different types of tyrosine kinase receptor inhibitors cannot only inhibit drug resistance to some extent but also reduce their toxicity. Currently, relevant clinical trials have been conducted on malignant tumours such as melanoma (Robert et al. 2015). In addition, Hussain et al. (2019) believed that interfering with the signalling pathways involved in targeted therapy may be appropriate in reducing drug resistance. For example, EGFR inhibitors can block the mediation of EGFR in the PI3K/AKT and RAS/ERK pathways; these two pathways interact in autoregulation, which may play a crucial role in reducing drug resistance.

Minimisation of adverse reactions is another challenge that needs to be overcome. Targeted therapy often results in adverse drug reactions; although, the symptoms are usually not serious, some can be life-threatening (Udagawa and Zembutsu 2020). For example, the EGFR inhibitor trastuzumab can cause cardiotoxicity, and approximately, 5% of patients exhibited reduced left ventricular ejection fraction (Seidman et al. 2002). The most common complications of the VEGF inhibitor bevacizumab are hypertension, mucosal bleeding, and haematuria, which can lead to gastrointestinal perforation and be life-threatening in severe cases (Jia et al. 2021). Further research regarding novel target inhibitors and individualised selection of target drugs based on symptoms may be effective for the monitoring and prevention of drug reactions. Furthermore, studies have shown that targeted therapy combined with immunotherapy can reduce the severity and incidence of adverse reactions to a manageable level (Chau and Bilusic 2020; Taylor et al. 2020). However, no clinical trials have been conducted regarding the adverse reactions to osteosarcoma treatment; therefore, more research is still needed to prove its feasibility.

Summary

Targeted osteosarcoma therapy has been widely studied in recent years, owing to its high specificity and favourable side effect profile. However, problems, such as resistance and adverse reactions, may limit its application. This article reviews the current state of research regarding targeted osteosarcoma therapy. Targeted therapy shows potential in the treatment of osteosarcoma and may offer an important means of precise and personalised treatment in the future.

Author contributions

SL wrote the main manuscript, HZ and JL prepared the tables and figures, GS edited this manuscript and viewed this study. All the authors reviewed the manuscript.

Funding

(1) Shenyang Science and Technology Project (21-173-9-24). (2) China Postdoctoral Science Foundation, sponsor number: 2021M693912. (3) 345 Talent Project of Shengjing Hospital (M0944 and M0744).

Data availability

Not applicable.

Declarations

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.