KRASG12C is present in about 3% of metastatic CRC [16]. The potential to target KRAS has been revolutionized with selective KRASG12C inhibitors that bind to a pocket in the switch II region exposed in GDP-bound RAS. These drugs take advantage of the cysteine residue in the mutant protein and the intrinsic GTPase of KRASG12C, which is relatively higher than for other KRAS mutants [17]. The two KRASG12C inhibitors furthest along in clinical development are sotorasib (AMG510) and adagrasib (MRTX849).

Early data suggest response rates of 7%–20% for KRASG12C inhibitor monotherapy in patients with metastatic KRASG12C CRC. The phase 1 clinical trial of sotorasib had a 7% response rate (3 of 42 patients), 73.8% disease control rate (DCR) and median PFS of 4.0 months [18]. The subsequent phase 2 trial of sotorasib at 960 mg daily reported objective response in 6 out of 62 (9.7%) patients but did not meet the primary endpoint of ORR of 20%. DCR was 82.3%; median PFS and OS were 4 and 10.6 months, respectively. The phase 2 CRC cohort of the KRYSTAL-1 study of adagrasib had an ORR of 19% (in 8 patients) and DCR of 86% (in 37 patients) among 43 evaluable patients. Median PFS was 5.6 months [19]. This clinical activity of KRASG12C inhibitors in metastatic CRC is lower than that seen in metastatic non-small cell lung cancer, for which sotorasib has been granted accelerated approval by FDA based on results of phase 2 trial [20].

Several research groups have shown reactivation of RAS signaling through RTKs to be a key mechanism of resistance to KRASG12C inhibition in CRC [21∗∗, 22, 23∗∗]. Amadeo et al. showed that in KRASG12C CRC cell lines, KRASG12C inhibition results in transient ERK inhibition, after which there is phospho-ERK rebound. Compared to non-small cell lung cancer cells, the KRASG12C CRC cells have high basal RTK activation and are more responsive to growth factor stimulation, which induces higher level of phospho-ERK. The combination of cetuximab (EGFR inhibitor) and sotorasib resulted in sustained inhibition of phospho-ERK, increased cell death rate, and inhibited growth of patient derived CRC organoids and xenografts [21]. Ryan et al. showed that multiple RTKs can drive feedback reactivation of wild-type RAS (including NRAS and HRAS) after KRASG12C inhibition and provided evidence for co-inhibition of convergent nodes such as SHP2 or MEK to overcome adaptive resistance [22,23]. Clinical trials of KRASG12C and SHP2 inhibitors are ongoing.

Consistent with the preclinical studies, clinical outcomes of patients with KRASG12C metastatic CRC improved with co-targeting of KRASG12C and EGFR in ongoing trials. Results reported from the phase 1b (n = 40) cohort of sotorasib with panitumumab combination demonstrated ORR of 30% and DCR of 93% [24]. Median PFS was 5.7 months. Similarly, in the KRYSTAL-1 study phase 1b expansion cohort, the addition of cetuximab to adagrasib improved ORR to 46% (13 patients) and DCR to 100% among 28 evaluable patients; median PFS was 6.9 months. Treatment related adverse events of grade 3/4 were seen in 16% of patients [19]. These results have paved the way for registrational phase 3 clinical trials in patients with KRASG12C metastatic CRC. The KRYSTAL-10 trial (NCT04793958) randomizes patients to adagrasib (600 mg twice a day) with cetuximab (500 mg/m2 every 2 weeks) versus chemotherapy (mFOLFOX6 or FOLFIRI ± anti-VEGF/VEGFR) after progression on first line fluoropyrimidine-based doublet regimen. CodeBreaK300 (NCT05198934) is comparing the combination of sotorasib (960 mg daily in Arm A and 240 mg daily in Arm B) with panitumumab (6 mg/kg every 2 weeks) versus chemotherapy (Trifluridine and Tipiracil or Regorafenib) in the third-line setting. In contrast with the higher efficacy of this combination, co-inhibition of KRASG12C and MEK with sotorasib and trametinib combination among 18 patients with KRASG12C metastatic CRC achieved ORR and DCR of 11% (2 patients) and 83% (15 patients), respectively, and is not being further pursued [25]. Further combination regimens that are being investigated in metastatic CRC in the CodeBreaK101 study include triplet regimens, such as, sotorasib + trametinib + panitumumab, sotorasib + panitumumab + chemotherapy, and sotorasib + bevacizumab + chemotherapy.

A mechanism of intrinsic resistance to targeted therapy may be co-occurrence of other genomic alterations in the tumor that sustain oncogenic signaling through the same or different pathways when the driver oncogene is inhibited [26]. Over a quarter of patients with KRASG12C CRC have activating alterations in the PI3K/mTOR pathway, and 8% of patients have other likely pathogenic co-alterations in ERK signaling (e.g., mutations in BRAF, RAF1, HRAS, NRAS, MAP2K1, PTPN11, and other mutations in KRAS) [27]. Data from the adagrasib monotherapy and combination cohorts showed no association between PIK3CA mutation status and response, but analysis was limited by the small sample size [19]. Furthermore, CRISPR screens in lung and pancreatic cancer models treated with KRASG12C inhibitor have revealed collateral dependencies on genes in cell cycle, RTKs that promote target engagement upstream of KRAS, and parallel PI3K signaling pathway [28]. Hence, combinations of sotorasib with everolimus (mTOR inhibitor), sotorasib with palbociclib (CDK 4/6 inhibitor), and adagrasib with palbociclib are also being investigated in advanced solid tumors. Other KRASG12C inhibitors in development include GDC-6036 (alone and in combination with inhibitors of EGFR, VEGF, PI3Kα, SHP2), JDQ443 (alone and with co-inhibition of SHP2, EGFR, MEK, CDK4/6, anti-PD-1), JAB21822 (alone and with cetuximab), LY3537982 (alone and with co-inhibition of SHP2, EGFR, ERK1/2, AurA, CDK4/6, anti-PD-1).

Early studies of progression samples suggest multiple resistance alterations can emerge with KRASG12C inhibition in CRC and primarily converge to reactivate ERK signaling. Data from the 74 gene circulating tumor DNA assay (Guardant360) from baseline and progression samples collected from 45 CRC patients treated with sotorasib in CodeBreaK100 study [29,30] revealed detectable acquired genomic alterations in 32 patients (71%), involving RTK genes in 27% (including EGFR, ERBB2, KIT, ROS1, FGFR1, FGFR1, MET, and PDGFRA), cell cycle genes in 22%, DNA damage repair genes in 22%, and secondary RAS alterations in 16%. Genomic mechanisms of acquired resistance were also evaluated in 10 patients with metastatic CRC treated with adagrasib monotherapy on KRYSTAL-1 study using next generation sequencing of tumor tissue and or circulating tumor DNA [31]. Six patients had at least one putative resistance mechanism and five patients had multiple resistance alterations. Secondary KRAS mutations within the drug binding pocket (H95Q, H95R) were noted in two patients, other activating KRAS alterations in three and MAP2K1 alterations in 4 patients. KRASG12C amplification, NRASQ61K, BRAFV600E, and likely oncogenic PIK3R1S361fs and PTENN48K mutations were noted in one patient each. Three patients developed acquired gene fusions (EML4-ALK rearrangement; CCDC6-RET fusion; and multiple fusions involving FGFR3, BRAF, and RAF1). A functionally distinct KRASG12C inhibitor, RM-018, that forms a tricomplex (RM-018, cyclophilin A, GTP-bound KRASG12C), was able to overcome resistance due to acquired KRASY96D mutation affecting the switch–II binding pocket in cell lines from KRASG12C lung and pancreatic cancers [32].