Recent advances on anti-angiogenesis receptor tyrosine kinase inhibitors in cancer therapy

As described by Hanahan and Weinberg, tumor angiogenesis is regarded as one of the ten hallmarks of cancer [1]. Neovasculature supplies tumor with essential nutrients and oxygen and removes waste produce especially for those whose size is larger than 1–2 mm [2]. Based on the theory that tumor growth, progression, and metastasis depend on angiogenesis, targeting tumor blood vessels has been introduced as a logical approach to the treatment of various malignancies [3]. Subsequently, enormous innovative anti-angiogenic agents have been developed and tested in clinical trials.

In physiological circumstances, angiogenesis is under a relatively dynamic homeostasis, tightly controlled by pro-angiogenic and anti-angiogenic regulators. In cancer, however, the balance of pro- and anti-angiogenic are disturbed, leading to the switch to angiogenesis [4]. Tumor angiogenesis is an intricate mechanism regulated by multiple signaling pathways. Certain well-known pro-angiogenic factors include vascular endothelial growth factor (VEGF) [5], angiopoietin (ANGPT) [6], basic fibroblast growth factor (bFGF) [7], and platelet-derived growth factor (PDGF) [8], while endogenous anti-angiogenic factors encompass endostatin [9], angiostatin [10], and so on. In the past few decades, the efforts to develop anti-angiogenic treatment mainly focus on inhibiting VEGF/VEGFR signaling pathway such as anti-VEGF antibody bevacizumab [11] and anti-VEGFR2 antibody ramucirumab [12]. Anti-VEGF/VEGFR single-target drugs often lead to transient responses; however, tumor progression happens because other pathways, such as the PDGF/ PDGFR, FGF/FGFR, and ANGPT/Tie-2, provide potential escape mechanisms [13]. Anti-angiogenic agents inhibiting multiple signaling pathways seem more promising; therefore, multiple pan-target agents have been developed [13].

The binding of ligand to its receptor, for instance, VEGF and VEGFR, initiates the activity of tyrosine kinase domain of receptor and upregulates the downstream signal system. Those kinases are often upregulated in cancer and regarded as attractive therapeutic candidates. By April 2017, among the 35 protein kinase inhibitors (PKIs) approved, 31 are used in cancer therapy [14]. For example, sorafenib, sunitinib, and pazopanib are approved anti-angiogenic small molecule tyrosine kinase inhibitors (TKIs). Several novel anti-angiogenic TKIs with promising preclinic outcome are being studied in phase III clinical trials. Furthermore, new advance on combination of immunotherapy and TKIs attracts attention for superior efficacy.

There are 518 protein kinases encoded in the human genome, of which 90 kinases belong to the group of tyrosine kinases, being a major subclass of the human protein kinases [15, 16]. The tyrosine kinase group can be classified in receptor tyrosine kinases (RTKs) and non-receptor tyrosine kinases (nRTKs), and the former exerts function on the transduction of extracellular signals into the cell while the latter accomplishes intracellular communication [17]. RTKs share a great degree of similarity in their molecular structure, with an extracellular domain which can bind specific ligands, a single trans-membrane helix, and an intracellular region which contain a protein tyrosine kinase domain [18]. The intracellular domain, also called kinase domain, which is a bi-lobar structure, is composed of an N-terminal lobe, a C-terminal lobe, and an adenosine triphosphate (ATP) binding cleft located between them [19]. Ligand binding to the extracellular domain induces dimerization and allows auto-phosphorylation of the intracellular domains and activation of the receptor’s tyrosine kinase [20]. RTKs catalyze the transfer of the phosphates of ATP to the hydroxyl group of tyrosine residues on target proteins [21]. The RTK family includes the insulin receptor and the receptors for many growth factor families such as VEGF, FGF, PDGF, and epidermal growth factor (EGF) [21]. The VEGF-related gene family comprises six secreted proteins, namely VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, and placenta growth factor (PIGF) [22], and the VEGFR family consists of three related RTKs, VEGFR-1, VEGFR-2, and VEGFR-3 (Fig. 1) [23]. VEGF-A is the most important mediator, mediating its effects by binding to its two high-affinity RTKs: VEGFR-1 and VEGFR-2. PDGFs are the second important growth factor related to angiogenesis. There are at least four members in the PDGF family, namely PDGF-A, PDGF-B, PDGF-C, and PDGF-D. PDGFs act via two RTKs, known as PDGFR-α and PDGFR-β [24]. Meanwhile, bFGF, belonging to the FGF family, also contribute to angiogenesis [25]. The binding of ligand to its corresponding receptor initiates phosphorylation of the RTKs and leads to the activation of downstream signaling pathways, such as the PI3K/Akt and Ras/Raf/MEK/MAPK, which are involved in the proliferation, migration, and apoptosis of endothelial cells [26]. Therefore, small molecule inhibitors of RTKs are regarded as rational targets for cancer therapy.

Currently, angiogenic TKIs approved to cancer treatment by the US Food and Drug Administration (FDA) or China Food and Drug Administration (CFDA) are listed in Table 1.

While the approved anti-angiogenic TKIs are trying to expand their indication in other cancer types, numerous new anti-angiogenic TKIs are also being extensively explored. Three representative TKI drugs with potential to be approved in the near future are presented.

Motesanib, also named AMG 706, is an orally multi-targeted inhibitor of VEGFR-1/2/3, PDGFR-α/β, and c-Kit [82]. Motesanib was considered as a potent anti-tumor drug in Asian advanced NSCLC patients based on the subgroup analysis of MONET1 trial [83]. However, the results of later phase III trial (MONETA) were disappointing with no advantage in patients receiving motesanib plus paclitaxel and carboplatin over placebo plus paclitaxel and carboplatin [84]. Nevertheless, two phase II trials have indicated remarkable anticancer activity of motesanib among patients with advanced thyroid cancer [85, 86]. Recently, Lubner et al. examined the efficacy of motesanib in low-grade NETs in a phase II trial [87]. The study reached its primary objective with a 4-month PFS of 78.5%, and the median PFS in all patients was 8.7 months. All in all, motesanib is as potential as a systemic targeted therapy for NETs, but its niche in the treatment of NETs still needs further study.

Cediranib, also named AZD-2171, is a potent inhibitor of VEGF signaling that binds all three VEGFR (VEGFR-1/2/3), together with c-Kit and PDGFR-α/β [88]. Though, cediranib had failed phase III clinical trials in NSCLC [89], mCRC [90, 91], and recurrent glioblastoma [92], it showed new hope in recurrent ovarian cancer. The ICON6 trial evaluated the efficacy and safety of cediranib plus platinum-based chemotherapy and as continued maintenance treatment in patients with relapsed platinum-sensitive ovarian cancer [93]. Unfortunately, the ICON6 trail was prematurely terminated on account of the depressing results in other cancer types. Finally, a total of 486 women were randomly allocated to arm A (chemotherapy plus placebo 6 cycles and continued placebo), arm B (chemotherapy plus cediranib 6 cycles and switched to placebo), and arm C (chemotherapy plus cediranib 6 cycles and continued cediranib). The median PFS was significantly longer in arm C at 11 months, compared to 8.7 months in arm A (p < 0.0001) [93], and the OS was 7.4 months higher in arm C compared to arm A (p = 0.21) [94]. Most common side effects of grade 3–4 in arm C were neutropenia, fatigue, and hypertension during the chemotherapy phase and diarrhea, fatigue, and neutropenia during maintenance treatment.

Sulfatinib is a multi-target TKI targeting VEGFR-1/2/3, FGFR1, and colony stimulating factor 1 receptor (CSF1R). A phase I study (NCT02133157) observed an acceptable safety profile and encouraging antitumor activity in patients with advanced solid tumors, particularly in NETs [95]. At present, one phase II study (NCT02267967) and two phase III studies (NCT02589821, NCT02588170) conducted on advanced NETs are ongoing [96].

Immunotherapy has been changing the paradigm of oncology treatment in the recent years [97‐99]. Whether the combination of TKIs and immunotherapy can create synthetic effect is a hot topic. The emerging evidences suggest that anti-angiogenic therapy may not only inhibit neo-vascular formation, but also regulate the immune microenvironment [100]. This provided a theoretic basis for the combination of TKIs and immunotherapy. Subsequently, hundreds of clinical trials were designed to access the efficacy of combining TKIs with immune checkpoint blockade. A phase Ib study (JAVELIN Renal 100) conducted by Choueiri et al. interrogated the combination therapy of axitinib plus avelumab (a PD-L1 mAb) in advanced RCC patients [101]. The DCR reached 78% in 55 patients with three complete responses and the safety profile was manageable. These encouraging results supported the further study of these drug combinations. Now, the phase III JAVELIN Renal 101 trial finished [102]. The result showed that in 866 patients with mRCC, the axitinib plus avelumab group showed a remarkable improvement in median PFS compared with sunitinib (13.8 vs. 8.4 months, p < 0.0001), and the ORR were 51.4% and 25.7%, respectively. Furthermore, among patients with PD-L1+ tumors, the ORR and the PFS also favored the combination group [102]. The combination of axitinib and avelumab would be a promising strategy for patients with mHCC based on the positive result of JAVELIN Renal 101. Other combinations such as lenvatinib plus pembrolizumab or SHR 1210 plus apatinib in patients with HCC were also ongoing [103]. The combination of immunotherapy with TKIs has demonstrated promising outcome in a certain type of carcinomas, but further optimized combinations are required and caution must be taken to avoid severe toxicity.

The development of anti-angiogenic agents has attracted great attention. Bevacizumab, the first clinically approved anti-VEGF targeted agents, provides a first proof of principle of anti-angiogenic treatment in cancer. Though monotherapy with bevacizumab is largely inefficient, it really exerts therapeutic efficacy in various types of carcinoma when in combination with chemotherapy [104]. Because tumor angiogenesis is regulated by multiple pathways, many interconnected pathways can compensate the effect of single inhibition of VEGF signaling. It seems that multi-targeted TKIs hold a therapeutic advantage over monoclonal antibody as they can block multiple angiogenic signaling pathways simultaneously. Indeed, TKIs have shown their efficacy in many types of cancers, mainly RCC and HCC. Although all anti-angiogenic receptor TKIs share the same mechanism of action and the similar spectrum of targeted kinases, they are different in their pharmacokinetics and substance-specific AEs. The one possible explanation may be that the subtle difference on chemical structure leads to the variable affinity and potency to targets. Another possibility is that those TKIs may act on some unidentified targets beyond known kinases. With more and more anti-cancer agents available, it is a challenge for the oncologist to make an optimal choice in the sequence of treatment. For instance, 12 drugs have been approved for patients with HCC, including 6 anti-angiogenic TKIs until 2017 [105]. Though, the international guidelines have reached a global consensus for the choice of drugs in different lines. The optimal strategy and the sequence of drugs as well as the right time of the incorporation of other therapeutic methods such as surgery, radiology has not yet been resolved. Tolerance of receptor TKIs should also be taken into account.

Another challenge for anti-angiogenesis TKIs is the lack of robust biomarkers to identify patients with cancer who will benefit from anti-angiogenic therapy. Unlike RTK inhibitor, larotrectinib is special for cancer with tropomyosin receptor kinases (TRK) fusion-positive and has demonstrated significant efficiency in patients with different tumor histology [106]. One of the main problems in identifying such a biomarker for anti-angiogenic therapy may come from the complex feedback loops and cross talk between signaling pathways. Currently, some biomarkers have been proposed, such as VEGF, VEGFR-2, FGF-2, or IL-8, but none of them have yet been validated for routine clinical use [104]. Recently, a cohort study conducted by Liu et al. indicated a positive correlation between the anti-angiogenesis-related AEs and prolonged OS [107]. It means that side effects, such as high blood pressure, hypothyroidism, or hand-foot syndrome, may associate with the anti-tumor efficacy. Similarly, Rini et al. demonstrated that patients with diastolic blood pressure ≥ 90 mmHg had a longer OS and PFS [108]. As there are no molecular biomarkers available for clinical use, those side effects might be helpful for clinical decision.

The future of TKIs could be their positioning besides metastatic setting, such as in adjuvant therapy and neoadjuvant treatment. There were three well-known phase III clinical trials that explored the use of TKIs in RCC in adjuvant setting, namely ASSURE (adjuvant sunitinib vs. sorafenib vs. placebo), PROTECT (pazopanib vs. placebo), and S-TRAC (sunitinib vs. placebo) [109‐111]. Only S-TRAC study showed a significant improvement by sunitinib in disease-free survival in high-risk RCC after nephrectomy [111]. Based on the result of S-TRAC trial, sunitinib was approved by the FDA as an adjuvant therapy for RCC patents in 2017. Unfortunately, adjuvant sorafenib for HCC patients reached a negative result [112]. The utilization of TKIs before surgery has also been studied. A phase II trial explored the safety and efficacy of the use of pazopanib prior to cytoreductive nephrectomy RCC patients, suggesting the safety, and clinical benefit could be expected [113]. The precision role of anti-angiogenic TKI in adjuvant and neoadjuvant therapy needs further investigation.

It is noted that the indication of these receptor TKIs are mainly restricted to highly vascular tumor, like RCC, HCC, NSCLC, and CRC. Their efficacy in other types of cancers needs further exploration [30]. In most case, anti-angiogenesis treatment increases the PFS of patients, while the increase in OS is unsatisfactory. Great breakthrough in immunotherapy brings new possibility for the combination of TKIs, and positive results in a certain type of carcinoma attract broad attention [114].