Tarlox and Sotorasib in Patients With KRAS G12C Mutations

KRAS acts as a key protein in transducing signals from cell surface receptors, such as receptor tyrosine kinases (RTKs) into cells to initiate a network of cytoplasmic and nuclear signaling cascades that mediate key processes, such as cell cycle entry and cell survival that regulate normal tissue homeostasis. Due to the central importance of KRAS in mediating critical cellular processes, there are significant interconnected signaling feedback pathways that protect normal cells from uncontrolled proliferation and cell death. These signaling feedback pathways are also activated by tumor cells upon inhibition of mutated KRAS, which may result in intrinsic or acquired resistance to small molecule KRAS G12C inhibitors. Pre-clinical studies in non-small cell lung cancer (NSCLC) have shown that a key pathway that may be activated upon KRAS inhibition is the upstream ERBB RTKs, which provides the rationale for dual KRAS and ERBB blockade in overcoming resistance mechanisms in KRAS mutated NSCLC.

Lung cancer is the second most common cancer and the leading cause of cancer death in the United States. There were approximately 247,270 new cases of lung cancer that occurred in 2020. Prior studies have reported that lung cancer resulted in more deaths than breast cancer, prostate cancer, colorectal cancer, and leukemia combined in men ≥40 years old and women ≥60 years old. The past decade has seen a revolution of new advances in the management of non-small cell lung cancer (NSCLC) with remarkable progresses in screening, diagnosis, and treatment. The advances in systemic treatment have been driven primarily by the development of molecularly targeted therapeutics, immune-checkpoint inhibitors and anti-angiogenic agents, all of which have transformed this field with significantly improved patient outcomes. Despite these advances, most patients with advanced NSCLC have incurable disease, particularly after failure of a platinum-based chemotherapy regimen and check point inhibitors.

One of the earliest identified molecular drivers of NSCLC is the GTPase transductor protein called KRAS. It is a member of the RAS family of oncogenes and at the apex of multiple signaling pathways central to tumor cell proliferation. KRAS mutant lung cancers have worse outcomes in both early stage and advanced metastatic settings, illustrating the critical need for novel agents targeting KRAS-driven NSCLC. KRAS G12C mutations are present in ~15% of lung adenocarcinomas and 0-8% of other cancers. The missense mutation at codon 12 interferes with the GTPase, activating protein mediated GTP hydrolysis and shifting the equilibrium between the signaling-competent KRAS-GTP and signaling incompetent KRAS-GDP in favor of the GTP bound state. This process links upstream cell surface receptors such as the ERBB family (EGFR, HER-2, HER-3, HER-4) to downstream pathways such as RAF/MEK/ERK and PI3K/AKT/mTOR which leads to uncontrolled cell proliferation and survival.

Attempts to identify small molecular inhibitors of KRAS have been unsuccessful for many years as there was a lack of a clearly defined deep pocket in the structure of RAS outside of the nucleotide binding site and the challenge of targeting the nucleotide binding site due to extraordinarily high affinity of GTP. Recently, several pioneering studies have identified small molecule cysteine-reactive inhibitors that covalently modify the mutant KRAS G12C protein to reveal an allosteric switch II pocket. The induction of the structurally disordered pocket by these small molecule inhibitors converts the GTP preference of naïve KRAS G12C to the inactive GDP bound state, impairing its interaction with downstream effectors.

AMG510/Sotorasib was the first KRAS G12C inhibitor to the enter the clinic. A Phase I/II clinical trial involving 129 patients included 59 patients with KRAS G12C mutated NSCLC who had progressed on prior standard therapies. Patients were enrolled in dose escalation and expansion cohorts to receive daily sotorasib monotherapy (960 mg PO daily). At a median follow up of 12.2 months, approximately 50% of NSCLC patients demonstrated tumor regression with a confirmed objective response rate (ORR) of 37.1% (95% CI 28.6-46.2%) and a disease control rate (DCR) of 80.6% (95% CI 72.6-87.2%). The median time to objective response was 1.4 months and median duration of response was 10 months, with median progression free survival (PFS) of 6.8 months. The FDA has now accepted a new drug application and granted it a priority review for the treatment of patients with KRAS G12C mutant locally advanced or metastatic NSCLC following at least 1 prior systemic treatment, with an expected decision date by August 2021.

Although the results from these early-stage clinical trials showed promise, ~50% of KRAS G12C mutant NSCLC patients failed to respond to therapy and rate of relapse is high calling for the need for novel combinations to overcome intrinsic and acquired resistance mechanisms in this subset of patients.

It was previously thought that constitutively active oncogenic KRAS induces growth factor independence. However, recent evidence has suggested that specific KRAS mutant isoforms such as KRAS G12C may be regulated by upstream activation of several receptor tyrosine kinases. The pattern of RTK dependence appears to vary between KRAS G12C mutant cancer but numerous RTKs are involved in the adaptive feedback mechanism to G12C inhibition. This may be mediated through an adaptive RAS pathway activation. Silencing oncogenic KRAS in EGFR/HER dependent cells reduced cellular growth and induced a modest apoptotic signal. The depletion of KRAS expression by mutant specific siRNA was accompanied by a reduction in AKT phosphorylation in the ERBB dependent subset and activation of STAT3, which suggests a feedback loop via STAT3 that re-establishes oncogenic signaling thereby compensating for the loss of AKT survival signals. Pan ERBB inhibition in these KRAS G12C mutated NSCLC lines resulted in a potent suppression of growth and inhibition of receptor signaling and downstream signaling effectors. Thus, sole silencing of oncogenic KRAS may not be an effective therapeutic strategy in KRAS-addicted cancers since upstream events and feedback loops are likely to attenuate or annul the effects of the therapeutic intervention.

These pre-clinical studies provide a solid ground to evaluate the use of pan-ERBB inhibitors in combination with KRAS G12C inhibitors in patients with KRAS mutant lung tumors.

Tarloxotinib is a recently discovered novel prodrug that releases a potent, irreversible pan-ERBB (EGFR, HER2 and HER4) TKI. It is designed to be inactive under normal oxygen conditions but undergoes fragmentations under low oxygen conditions (hypoxia) to release the potent irreversible active metabolite (tarloxotinib-E) that has activity against both normal and mutant versions of the ERBB family. Selective production of tarloxotinib-E under hypoxic conditions generates a therapeutic window where it is selectively activated in hypoxic tumor regions to deliver higher drug delivery to tumor tissue. This reduces systemic exposure which avoids on-target EGFR related toxicities than standard EGFR TKI. In a mouse xenograft model of a human derived EGFR exon 20 insertion, intra-tumoral tarloxotinib-E levels were 20 time higher than skin and 50 times higher than plasma demonstrating selective tumor conversion. This strategy broadens the therapeutic window leading to improved efficacy, while reducing toxicity. Multiple pre-clinical studies have further demonstrated the efficacy of tarloxotinib compared to standard EGFR inhibitors.

Phase I clinical trials to determine the MTD and DLTs of tarloxotinib enrolled 27 patients with locally advanced or metastatic solid tumors. Of the patients that received tarloxotinib as a weekly 1-hour infusion, 6 patients received the drug at the recommended Phase 2 dose of 150 mg/m2 with good tolerance.

The combination of tarloxotinib with sotorasib is poised to provide highly specific tumor inhibition while targeting the vertical KRAS signaling pathway with minimal toxicity. The combination is unlikely to result in clinically relevant drug-drug interaction (DDI) based on absorption, metabolism, elimination or protein-binding. Tarloxotinib is intravenously administered while sotorasib is a small molecule that is administered orally; no absorption interactions are expected.