Abstract 5691: SM08502, a novel, small-molecule CDC-like kinase (CLK) inhibitor, demonstrates strong antitumor effects and Wnt pathway inhibition in castration-resistant prostate cancer (CRPC) models

Abstract

CRPC is associated with primary and acquired chemotherapy resistance. Loss of androgen receptor (AR) signaling or development of AR splicing variants (e.g., ARV7) in CRPC imparts resistance to standard of care (SOC) agents that target AR signaling (e.g., enzalutamide and abiraterone). Effective therapies are an unmet need for CRPC patients with treatment-resistant tumors. Aberrant Wnt pathway activation contributes to resistance to AR-targeted agents, and cytotoxic chemotherapies such as docetaxel have been shown to activate Wnt signaling in PC cells. SM08502 has demonstrated strong antitumor activity in several preclinical cancer models and has been shown to inhibit the Wnt signaling pathway via disruption of alternative splicing. Here, we examined the antitumor activity of SM08502 in preclinical models of CRPC. The effect of SM08502 on cell proliferation was tested in 5 PC cell lines. Proliferation was strongly impaired by SM08502 across all cell lines (average EC50=0.319 μM [0.191–0.462]) irrespective of their mutation profile or hormone sensitivity. Compared to DMSO, SM08502 inhibited serine/arginine-rich splicing factor 6 (SRSF6) phosphorylation and potently suppressed Wnt-related gene (LRP5, TCF7, TCF7L1) and protein expression. In vivo antitumor effects and tolerability of QD oral SM08502 were assessed in multiple xenograft models, including mice bearing 22RV1 (ARV7+) or PC3 (AR-/-) CRPC flank xenografts (n=6/group). In 22RV1 xenografts, tumor growth inhibition (TGI) was demonstrated in mice treated with 12 and 25 mg/kg SM08502 (35%, P<0.05; 73%, P<0.001, respectively) vs. vehicle at D24 of treatment. In PC3 xenografts, significant TGI was seen in mice treated with 25 mg/kg SM08502 (75%, P=0.03) vs. vehicle. In 22RV1 xenografts, no TGI was observed with 75 mg/kg abiraterone or 30 mg/kg enzalutamide treatment (-4% and 12%, respectively) vs. vehicle, which confirmed the effect of ARV7 on resistance. Similarly, no significant TGI was seen in PC3 xenografts treated with enzalutamide, abiraterone, or docetaxel (-52%, P=0.2; -60%, P=0.33; 26%, P=0.38, respectively). No combination effect on TGI was observed with 25 mg/kg SM08502 + docetaxel (90%, P<0.05), but more tumor regressions occurred with combined treatment (4/6) than SM08502 alone (2/6). SM08502 was well tolerated in all tested xenograft models (<15% bodyweight loss from baseline). In summary, SM08502 potently inhibited cell proliferation, SRSF6 phosphorylation, and Wnt-related gene expression in multiple PC cell lines. In vivo, SM08502 demonstrated strong antitumor effects in CRPC xenografts. These data suggest that SM08502 has the potential to provide clinical benefit to patients with treatment-resistant CRPC. A Phase 1 study of SM08502 in subjects with advanced solid tumors is ongoing (NCT03355066). Citation Format: Carine Bossard, Nathalia Cruz, Kevin Chiu, Brian Eastman, Chi Ching Mak, Sunil KC, Gail Bucci, Josh Stewart, Timothy J. Phalen, Steven Cha. SM08502, a novel, small-molecule CDC-like kinase (CLK) inhibitor, demonstrates strong antitumor effects and Wnt pathway inhibition in castration-resistant prostate cancer (CRPC) models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5691.

Development of Cdc2-like Kinase 2 Inhibitors: Achievements and Future Directions

“…The isoquinoline-based inhibitor 4 (SM08502, Table ) is another approach by Biosplice, developed through structure-based drug design. It has entered phase 1 trials as an oral pan-CLK inhibitor (CLK2 IC 50 = 2 nM) for the treatment of advanced solid tumors. , Notably, CLK3 (IC 50 = 22 nM) was also inhibited by compound 4 . Recently, Biosplice reported that indazole-3-carboxamide derivative 5 (SM04755, Table , Figure A) guided by a structure–activity relationship (SAR) exhibited picomolar activity in enzymatic assays (CLK2 IC 50 = 0.822 nM) and lower nanomolar activity in cellular assays (cell CLK2 EC 50 = 268.1 nM). − Docking studies suggested that 5 formed hydrogen bonds with Glu244, Leu246, and Lys193 in the ATP-binding site of CLK2 .…”

“…CLK2 inhibitors were first described in 2009; in recent years, attempts have been made to discover and investigate potential selective CLK2 inhibitors. ,,, Although several CLK2 inhibitors have been reported in literature and patents, only a few compounds have entered clinical trials. To the best of our knowledge, two CLK2 inhibitors and three pan-CLK inhibitors are currently under investigation in clinical trials for the treatment of degenerative disease and tumors.…”

“…78 It is suitable for HTS and offers advantages such as low loss of protein and high temperature ranges. 42 The results of this method are easily 33,63,66,79 Although several CLK2 inhibitors have been reported in literature and patents, only a few compounds have entered clinical trials. To the best of our knowledge, two CLK2 inhibitors and three pan-CLK inhibitors are currently under investigation in clinical trials for the treatment of degenerative disease and tumors.…”

“…Alternative splicing is a critical molecular mechanism of gene expression in which pre-mRNA transcripts from a single gene are spliced into multiple isoforms. , CLKs are involved in the assembly of spliceosomes and are implicated in both constitutive and alternative splicing control mechanisms and the selection of splicing sites. − They consist of four characterized isoforms in mammals (namely, CLK1, CLK2, CLK3, and CLK4) and belong to the CMGC group of kinases which include cyclin-dependent kinases (CDKs), CLKs, mitogen-activated protein kinase (MAPK), GSK3, serine–arginine-rich protein kinases (SRPK), and dual-specificity tyrosine phosphorylation-regulated kinases (DYRK). , The overexpression of CLK proteins localizes to nuclear speckles, where splicing factors are concentrated and affects slicing site selection of pre-mRNA . Several small-molecule inhibitors of the CLK family have been reported to control mRNA splicing. , …”

“…32 Several small-molecule inhibitors of the CLK family have been reported to control mRNA splicing. 33,34 Among the CLK family, CLK2 has been recently reported to be a promising drug target for various diseases. 6,35 Hence, there is a demand for developing CLK2 modulators on various disease settings.…”