The novel pyrrolo-1,5-benzoxazepine, PBOX-15, synergistically enhances the apoptotic efficacy of imatinib in gastrointestinal stromal tumours; suggested mechanism of action of PBOX-15

Abstract

The C-KIT receptor tyrosine kinase is constitutively activated in the majority of gastrointestinal stromal tumours (GIST). Imatinib (IM) a selective inhibitor of C-KIT, is indicated for the treatment of KIT-positive unresectable and/or metastatic GIST, and has tripled the survival time of patients with metastatic GIST. However, the majority of patients develop IM-resistance and progress. Although IM elicits strong antiproliferative effects, it fails to induce sufficient levels of apoptosis; acquired IM-resistance and disease recurrence remain an issue, a more effective drug treatment is greatly needed. We examined the effect of a novel microtubule-targeting agent (MTA), pyrrolo-1,5-benzoxazepine (PBOX)-15 in combination with IM on GIST cells. PBOX-15 decreased viability and in combination with IM synergistically enhanced apoptosis in both IM-sensitive and IM-resistant GIST cells, decreased the anti-apoptotic protein Mcl-1, and enhanced activation of pro-caspase-3 and PARP cleavage. The combination treatment also led to an enhanced inhibition of C-KIT-phosphorylation and inactivation of C-KIT-dependent signalling in comparison to either drug alone; CDC37, a key regulator of C-KIT in GIST was also dramatically decreased. Furthermore, PBOX-15 reduced CKII expression, an enzyme which regulates the expression of CDC37. In conclusion, our findings indicate the potential of PBOX-15 to improve the apoptotic response of IM in GIST cells and provide a more effective treatment option for GIST patients.

1,5‐Benzoxazepines as a unique and potent scaffold for activity drugs: A review

“…[38] Moreover, compound 32 influences the healing effect of imatinib used in gastrointestinal stromal tumors. [39] Overall, this study showed for the first time the PBOX-6-induced reactive oxygen species the molecular and cellular mechanisms in PBOX-6-induced cell death in neuroblastoma. [40] The same group of researchers designed and synthesized a series that drives colon tumorigenesis.…”

Role of protein kinase CK2 in antitumor drug resistance

“…The co-chaperone CDC37, which has a prominent role on the onco-kinome, is also controlled by CK2 [79]. The suppression of CDC37 phosphorylation and stabilization by CK2 was recently identified in the mechanism exploited by the microtubule-targeting pyrrolo-1,5-benzoxazepine compound for overcoming resistance to imatinib of gastrointestinal stromal tumor cells [21].…”

“…CDC37 is another CK2 substrate critically important in TKI resistance: in imatinib-resistant gastrointestinal stromal tumors (GIST) cells with constitutively active c-KIT receptor tyrosine kinase, a mechanism has been described where CK2 has a role through the phosphorylation of CDC37, which in turn, in a positive loop, contributes in maintaining high levels of CK2 itself. In fact, treatments with PBOX-15 (pyrrolo-1,5-benzoxazepine, a microtubule-targeting compound), which reduce both CDC37 and CK2 levels, increase sensitivity to imatinib [21]. In CML bone marrow samples derived from patients resistant to imatinib, due to BCR-ABL T315I point mutation, a pro-apoptotic effect of CK2 inhibition was reported, mediated by PTEN reactivation [85].…”

Pre-clinical evaluation of a novel class of anti-cancer agents, the Pyrrolo-1, 5-benzoxazepines.

“…It is noteworthy that PBOX-induced apoptosis can also occur independent of change to Bcl-2 32, 21. To date, PBOX-6- and PBOX-15-induced apoptosis associates with a decrease in Mcl-1 expression 21, 23, 24, 27, 28. SiRNA-mediated downregulation of Mcl-1 augmented PBOX-6-induced apoptosis, suggesting that downregulation of Mcl-1 enhances or facilitates PBOX-6-induced apoptosis, rather than being the effective target 16.…”

“…A more favourable approach to overcome P-gp-mediated resistance is to administer agents which are not substrates for P-gp, such as the PBOXs 6, 24. Furthermore, PBOXs can bypass mutations in Bcr-Abl, p53 and various genetic anomalies in CLL and also, overexpression of Bcl-2, c-kit, P-gp, breast cancer resistance protein (BCRP) and v-myc avian myelocytomatosis viral oncogene (MYCN); to potently induce apoptosis in chemotherapy resistant cells 6, 12, 14, 17, 24, 27, 32. In summary, the PBOXs offer many advantages over current MTAs used within the clinic including structural simplicity, increased tumour specificity and insensitivity to a variety of mechanisms of chemoresistance.…”