Next Steps for KRAS Inhibitors Focus on Tackling Resistance

Pasi A. Jänne, MD, PhD

The recent approval of sotorasib (Lumakras) for the treatment of patients with advanced KRAS G12C–mutant non–small cell lung cancer (NSCLC) marks a milestone for cancer therapy. After 4 decades of research into the oncogenic implications of KRAS mutations in human cancers, the FDA’s May 2021 accelerated approval of the KRAS G12C inhibitor represents the first successful therapy directed at a target long considered undruggable.^1,2

Having finally made some headway in 1 cohort of patients, investigators are focused on finding ways to broaden the impact of KRAS inhibition. They are describing mechanisms of resistance to these drugs and offering a number of potential paths forward, including exploring coinhibitory targets (FIGURE²).^3-7

Figure. Potential Pathway Partners for KRAS G12C Inhibitors²

Many mechanisms of resistance hinge on reactivation of the RAS/MAPK pathway. Combinations of KRAS G12C inhibitors with drugs targeting upstream and downstream components of related signaling networks are showing promise in NSCLC and colorectal cancer (CRC).^8-11 Novel drug designs, such as RAS(ON) inhibitors, which target the active form of KRAS, could help tackle particularly challenging acquired resistance mutations that drive cross-resistance to KRAS G12C inhibitors.¹²

This year is shaping up to a busy one for KRAS research. Mirati Therapeutics’ adagrasib (MRTX849) is hot on the heels of sotorasib. The company expects to file a new drug application for the agent as a second-line treatment of patients with KRAS G12C–mutant NSCLC.¹³

A number of other companies are developing KRAS G12C inhibitors and vying for potential best-in-class status.^14,15 These drugs are being tested as monotherapy and in combination with a range of targeted therapies in KRAS G12C–mutant solid tumors (Table). Amgen is continuing its development of trend-setting sotorasib, notably in the CodeBreaK 101 trial (NCT04185883), a master protocol that currently includes 15 substudies.

Table. Select Clinical Trials of Combination Therapies in KRAS G12C–Mutant Solid Tumors

Investigators aim to recruit more than 1000 participants with KRAS G12C–mutant advanced solid tumors, NSCLC, or CRC.

Meanwhile, clinical testing is expected to start on the first KRAS(ON) agent and on drugs targeting other KRAS variants that are more common in malignancies other than NSCLC.

Resistance Mechanisms

Although KRAS G12C inhibitors offer an important new therapeutic option for a subset of patients with NSCLC, clinical trial experience suggests that fewer than half of patients respond and responses can be short-lived.^16,17 Furthermore, these drugs have modest activity as single agents in patients with CRC.³ In the phase 2 CodeBreaK 100 trial (NCT03600883), sotorasib monotherapy demonstrated an objective response rate (ORR) of 9.7% in 6 of 62 patients, all partial responses (PRs), according to peer-reviewing findings detailed in Lancet Oncology in December 2021. At the European Society for Medical Oncology (ESMO) Congress 2021, investigators reported that adagrasib monotherapy resulted in a 22% response rate, including 1 unconfirmed PR, among 45 patients treated during the phase 1/2 KRYSTAL-1 (NCT03785249) study.^9,18

Even preclinical studies have shown indications of variable degrees of sensitivity to KRAS G12C inhibitors.^19,20 Experience with other targeted therapies suggests that resistance to KRAS G12C inhibitors inevitably arises, and investigators have begun to unravel the underlying molecular mechanisms.

Studies indicate that rapid, adaptive RAS pathway reactivation is a key mechanism of resistance to KRAS G12C inhibitors, although it remains an open question whether this is mediated by activation of alternative wild-type RAS proteins (NRAS and HRAS), production of mutant KRAS G12C in the active guanosine triphosphate–bound state, or both.^2,3,21-23

In nonmutant cells, KRAS is maintained in the active state (which is not targetable by current KRAS G12C inhibitors) by the activity of upstream receptor tyrosine kinases (RTKs) and SHP2, a phosphatase that activates RAS signaling downstream of multiple RTKs.^21,22 Coinhibition of these targets may enhance the activity of KRAS G12C inhibitors.

Potential mechanisms of acquired resistance to KRAS G12C inhibitors were outlined by Pasi A. Jänne, MD, PhD, at the American Association for Cancer Research-National Cancer InstituteEuropean Organisation for Research and Treatment of Cancer (AACR-NCI-EORTC) Virtual International Conference on Molecular Targets and Cancer Therapeutics in October 2021.

Jänne, the 2021 Giants of Cancer Care^® award winner in the lung cancer category, is director of the Lowe Center for Thoracic Oncology, the Belfer Center for Applied Cancer Science, and the Chen-Huang Center for EGFR Mutant Lung Cancers at Dana-Farber Cancer Institute in Boston, Massachusetts.²⁴

During his presentation, Jänne noted that investigators are just beginning to understand these processes, with much of what is known having emerged over only the past several months.²⁴

He highlighted 2 key studies. In the first, investigators conducted molecular analyses of tumor tissue and circulating tumor DNA from plasma samples from 38 patients (27 with NSCLC, 10 with CRC, and 1 with cancer of the appendix) who exhibited acquired resistance following treatment with adagrasib in the KRYSTAL-1 study.⁴

A potential underlying cause of resistance was identified in 17 of the 38 patients. Seven of these patients displayed multiple concurrent mechanisms of resistance, which appeared to be more common in patients with CRC. Secondary alterations of the KRAS gene were identified in 9 of the 17 patients and included G12D/R/V/W, Q61H, R68S, H95D/Q/R, and Y96C mutations in addition to amplification of the KRAS G12C allele.⁴

Molecular alterations in non-KRAS genes involved in other RTK/RAS/ MAPK signaling pathways were observed in 12 of the 17 patients and included activating mutations in NRAS, BRAF, MAP2K1, and RET; gene fusions involving ALK, RET, BRAF, RAF1, and FGFR3; and loss-of-function mutations in NF1 and PTEN. Transformation from adenocarcinoma to squamous cell histology also was observed in 2 patients with NSCLC.⁴

In the second study, investigators used the chemical mutagen N-ethyl-N-nitrosourea to induce mutations in the KRAS gene, transduce the mutated genes into cell lines, and then study their impact on KRAS G12C inhibitor activity.⁶

Among clones resistant to sotorasib (n = 68), 52 harbored secondary KRAS mutations, most commonly A59T, R68M, and Y96D following high doses (≥ 1000 nM) of the drug, and G13D, A59S, R68M, and Q61L after lower doses (100nM-500 nM). Clones resistant to adagrasib (n = 74), 72 of which involved secondary KRAS mutations, most commonly harbored Y96D mutations after a high dose (≥ 200 nM) of adagrasib and Q99L, R68S, V8E, M72I, and A59S variants after low doses (20-100 nM).⁶

Investigators also evaluated the degree of resistance associated with each KRAS mutation. G13D, A59S/T, R68M, and Y96D/S mutations were all identified as highly resistant to sotorasib, whereas Y96D/S and Q99L conferred strong resistance to adagrasib. Some variants, such as V8E, G13D, A59S/T, and R68M that were resistant to sotorasib remained sensitive to adagrasib; the reverse was true for the Q99L mutation, which was resistant to adagrasib but remained sensitive to sotorasib. Notably, the Y96D/S mutations were identified as a mechanism of cross-resistance to both KRAS G12C inhibitors.⁶

Combination Therapy

Insights from these and other studies are already facilitating the design of novel therapeutic strategies and drugs that might prevent or delay the development of resistance.

Ongoing clinical trials of KRAS G12C inhibitors are largely focused on evaluating rational combinations based on positive preclinical results, including drug regimens cotargeting upstream components of the RAS pathway, including RTKs such as EGFR and HER, and downstream components such as MEK.

Following Jänne’s presentation at the 2021 AACR-NCI-EORTC conference were 2 reports of data from the ongoing phase 1b/2 CodeBreaK 101 trial, a multiarm study evaluating numerous sotorasib combinations. At the time of presentation, 41 patients (18 with CRC, 18 with NSCLC, and 5 with other solid tumors) had received treatment with sotorasib 960 mg daily in combination with the MEK inhibitor trametinib (Mekinist) at 1 mg or 2 mg daily in 1 arm of this study.

Among patients with CRC, the objective response rate (ORR) was 14.3% (95% CI, 0.4%57.9%) in those previously treated with a KRAS G12C inhibitor (n = 7) and 9.1% (95% CI, 0.2%-41.3%) for KRAS G12C inhibitor–naïve patients (n = 11). Among patients with NSCLC, ORRs were 0% in 3 patients and 20% in 15 patients (95% CI, 4.3%-48.1%), respectively. The disease control rates (DCRs) were 85.7% and 66.7% in patients with CRC and NSCLC who had previously received KRAS G12C inhibitors and 81.8% and 86.7%, respectively, among those who had not.¹⁰

The combination was well tolerated, with no unexpected or new toxicities and no fatal treatment-related adverse effects (TRAEs). The most common any-grade TRAEs were diarrhea, rash, dermatitis, nausea, vomiting, peripheral edema, decreased ejection fraction, and increased blood creatine phosphokinase. There was 1 dose- limiting toxicity (DLT): trametinib-related grade 3 maculopapular rash at the 2-mg dose.¹⁰

In a separate arm of the study, 33 patients were treated with sotorasib 960 mg daily in combination with the pan-ERBB inhibitor afatinib (Gilotrif) at 20 mg or 30 mg once daily (dose exploration) and afatinib 30 mg once daily (dose expansion), including patients who had previously received sotorasib.¹¹

Overall, the ORR was 30.3% (95% CI, 15.6%48.7%), consisting of all partial responses (PRs), and the DCR was 75.8%. ORRs were 20.0% (95% CI, 2.5%-55.6%) at the 20-mg dose and 34.8% (95% CI, 16.4%-57.3%) at the 30-mg dose. Among patients who had received prior sotorasib, 3 had stable disease (SD), 1 had progressive disease, and 1 withdrew due to an adverse effect.¹¹

The most common any-grade TRAEs included diarrhea, nausea, and vomiting. Diarrhea was also the most common TRAE of grade 3 or higher. One fatal TRAE, respiratory failure attributed to sotorasib, was reported in the 30-mg dose- exploration group.¹¹

At the 2021 ESMO Congress, investigators reported results from a third arm of CodeBreaK 101 in which the combination of sotorasib and EGFR-targeted antibody panitumumab (Vectibix) was evaluated.

The dose-exploration group (part 1 cohort A) comprised 8 patients who received sotorasib 960 mg daily plus panitumumab 6 mg/kg every 2 weeks, including 5 participants previously treated with sotorasib. In this cohort of 8 patients, there was 1 confirmed PR and 5 patients experienced SD. In the dose-expansion phase (part 2 cohort A), results were reported for 18 patients who were naïve to KRAS G12C inhibitor therapy and received the sotorasib/panitumumab combination at the recommended dose. There were 3 confirmed and 3 unconfirmed PRs plus 12 participants with SDs, for a DCR of 83.3%. Among the 41 enrolled patients, there were no DLTs or grade 4 or higher TRAEs.⁸

In another presentation at the 2021 ESMO Congress, investigators described data from a cohort of patients with CRC receiving adagrasib in combination with the EGFR antibody cetuximab (Erbitux) in the KRYSTAL-1 study. Among the 28 patients evaluable for response, the ORR was 43% including 2 unconfirmed PRs and the DCR was 100%. Grade 3/4 TRAEs were reported in 16% of patients, but there were no fatal events.⁹ The phase 3 KRYSTAL-10 trial (NCT04793958) of this combination in KRAS G12C–mutant CRC is ongoing.

Given the apparent complexity of KRAS G12C inhibitor resistance, in which alterations in multiple different RTKs are implicated,^4,22 the SHP2 protein, a central node in the RAS pathway that lies downstream of multiple different RTKs, has emerged as an attractive therapeutic target.² A number of SHP2 inhibitors are in development, several of which are being evaluated in combination with KRAS G12C inhibitors. BBP-398, a SHP2 inhibitor, is being evaluated in a phase 1 trial (NCT04528836) as monotherapy in patients with advanced solid tumors harboring KRAS G12C mutations or MAPK pathway alterations, excluding BRAF V600X mutations.

Tackling Cross-Resistance

Findings from studies of the mechanisms of acquired resistance to KRAS G12C inhibitors suggest that switching between these inhibitors upon progression could be effective against some KRAS resistance mutations. On the other hand, the Y96D/S variants appear to mediate cross- resistance to both drugs and present a particular therapeutic challenge.⁶

In the preclinical setting, combining KRAS G12C inhibitors with an inhibitor of the guanine nucleotide exchange factor SOS1 or with a SHP2 inhibitor was found to be active against the Y96D/S mutant.⁶ Several ongoing clinical trials are evaluating the combination of the SOS1 inhibitor BI 1701963 with KRAS G12C inhibitors.

The Y96D mutation also was shown to be susceptible to a new type of KRAS inhibitor that targets the GTP-bound form of the protein.⁸ Steve Kelsey, MD, MB ChB, FRCP, FRCPath, president of research and development at Revolution Medicines, which is developing these new drugs, described these KRAS(ON) inhibitors during a presentation at the 2021 AACR-NCI-EORTC conference.¹²

Based on the company’s proprietary tricomplex inhibitor technology, these drugs exploit cyclophilin A, a chaperone protein, to create a binding pocket for the RAS inhibitor on the smooth surface of the GTP-bound KRAS G12C protein. In addition, binding of the cyclophilin A protein to the active mutant protein blocks it from interacting with downstream effectors.¹²

A first-in-class KRAS G12C(ON) inhibitor, RMC-6291, is in preclinical development. The agent demonstrated a superior response rate compared with adagrasib in NSCLC KRAS G12C xenografts with an ORR per mouse RECIST of 68% (13 of 19 xenografts) vs 42% (8 of 19 xenografts), respectively.¹²

Beyond KRAS G12C

Revolution Medicines also is using its tricomplex technology to develop RAS(ON) inhibitors that target all KRAS variants. Although these pan-RAS(ON) inhibitors also have activity against wild-type RAS isoforms, meaning that they are likely to cause more toxicity than KRAS G12C inhibitors, they may offer another strategy for overcoming resistance.¹²

Lead candidate RMC-6236 has demonstrated antitumor activity in G12D-, G12V-, and G12Rmutant preclinical models, with notable activity against variants implicated in resistance to KRAS G12C inhibitors, according to Kelsey. Revolution Medicines plans to advance both RMC-6291 and RMC-6236 into clinical trials.¹²

Meanwhile, Mirati Therapeutics is developing MRTX1133, a KRAS G12D inhibitor with high affinity for both the active and inactive forms of the KRAS G12D–mutant protein. MRTX1133 has demonstrated potent antitumor activity in KRAS G12D–mutant xenograft models, with tumor regression in 60% of models and response in 73% of pancreatic cancer and 25% of CRC models.²⁵ Mirati expects to file an investigational new drug application for MRTX1133 in 2022.³

Jane de Lartigue, PhD, is a freelance medical writer based in Gainesville, Florida.

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