A bi-steric mTORC1-selective inhibitor overcomes drug resistance in breast cancer

Abstract

Activation of the PI3K-mTOR pathway is central to breast cancer pathogenesis including resistance to many targeted therapies. The mTOR kinase forms two distinct complexes, mTORC1 and mTORC2, and understanding which is required for the survival of malignant cells has been limited by tools to selectively and completely impair either subcomplex. To address this, we used RMC-6272, a bi-steric molecule with a rapamycin-like moiety linked to an mTOR active-site inhibitor that displays >25-fold selectivity for mTORC1 over mTORC2 substrates. Complete suppression of mTORC1 by RMC-6272 causes apoptosis in ER+/HER2− breast cancer cell lines, particularly in those that harbor mutations in PIK3CA or PTEN, due to inhibition of the rapamycin resistant, mTORC1 substrate 4EBP1 and reduction of the pro-survival protein MCL1. RMC-6272 reduced translation of ribosomal mRNAs, MYC target genes, and components of the CDK4/6 pathway, suggesting enhanced impairment of oncogenic pathways compared to the partial mTORC1 inhibitor everolimus. RMC-6272 maintained efficacy in hormone therapy-resistant acquired cell lines and patient-derived xenografts (PDX), showed increased efficacy in CDK4/6 inhibitor treated acquired resistant cell lines versus their parental counterparts, and was efficacious in a PDX from a patient experiencing resistance to CDK4/6 inhibition. Bi-steric mTORC1-selective inhibition may be effective in overcoming multiple forms of therapy-resistance in ER+ breast cancers.

Preclinical evaluation of the third-generation, bi-steric mechanistic target of rapamycin complex 1-selective inhibitor RMC-6272 in NF2-deficient models

“… 34 , 43 , 44 However, we did not detect induction of apoptosis, which differs from recent studies in several cancer types including bladder, renal, and breast where RMC-6272 treatment induced apoptosis, supporting a cell/tumor context-dependent process in NF2 -deficient meningiomas. 18 , 45 In addition, consistent with previous reports, we observed mTORC1 selectivity with effective inhibition of rapamycin-sensitive (S6K) and rapamycin-resistant (4E-BP1) substrates upon RMC-6272 treatment in primary and immortalized meningioma lines. It should be noted that in Ben-Men-1 cells, we initially detected inhibition of the mTORC2 substrate pAkt (S473) when treated with RMC-6272; however, upon lowering the treatment doses to the picomolar range, we observed mTORC1 selectivity in this cell line.…”

“… 10–12 Whereas the second-generation orthosteric inhibitor vistusertib, while attenuating S6K and 4E-BP1 and showing more promising results, was poorly tolerated in patients, 2 potentially due to targeting of mTORC2, which regulates several crucial metabolic processes. 17 , 18 Here, we have tested the potency of a third-generation inhibitor RMC-6272, a tool compound representative of the clinical investigational agent RMC-5552, in NF2 -deficient preclinical models. We observed superior inhibition of proliferation in both immortalized and primary meningioma lines upon RMC-6272 treatment, with IC50s in the picomolar range, compared to first- or second-generation mTOR inhibitors.…”

“… 2 The significant toxicity of second-generation inhibitors like vistusertib may be attributed to the targeting of mTORC2, which regulates several crucial functions, such as glucose homeostasis and lipid metabolism. 17 , 18 While these findings indicate the importance of the mTOR pathway in NF2-associated tumors, they also highlight the need for improved mTORC1-specific compounds.…”

“…Subsequently, improved bi-steric versions demonstrate increased selectivity by several-fold for mTORC1 over mTORC2, and have garnered recent attention for the treatment of several solid tumors, owing to their unique chemistry. 18 , 20–22 One such compound of this class is the investigational agent RMC-5552 which has over 30-fold selectivity for mTORC1 and is currently under clinical evaluation ( https://clinicaltrials.gov/ : NCT04774952 and NCT05557292). Based on the established importance of mTORC1 signaling in NF2 -deficient meningiomas, we have explored the potential of RMC-6272, a preclinical tool compound representative of the clinical investigational agent RMC-5552.…”

Unmasking the tumourigenic role of SIN1/MAPKAP1 in the mTOR complex 2

“…6,79 To date, there is no specific mTORC2 inhibitor available in the clinic; however several authors have shown the benefit of targeting it specifically. 26 Nevertheless, the importance of the mTORC2 in metabolism, 80,81 proliferation 51,53,70 cytoskeleton 82,83 and immune response 84,85 suggest that inhibiting SIN1 could impact the normal cellular functioning of non-tumour cells. However, by specifically targeting protein-protein interactions within mTORC2, it may be possible to selectively inhibit the oncogenic functions of this complex while reducing off-target effects, improving the safety profile compared to mTOR inhibitors.…”