Despite decades of research, efforts to directly target KRAS have been challenging. MRTX849 was identifi ed as a potent, selective, and covalent KRAS G12C inhibitor that exhibits favorable drug-like properties, selectively modifi es mutant cysteine 12 in GDPbound KRAS G12C , and inhibits KRAS-dependent signaling. MRTX849 demonstrated pronounced tumor regression in 17 of 26 (65%) KRAS G12C -positive cell line-and patient-derived xenograft models from multiple tumor types, and objective responses have been observed in patients with KRAS G12C -positive lung and colon adenocarcinomas. Comprehensive pharmacodynamic and pharmacogenomic profi ling in sensitive and partially resistant nonclinical models identifi ed mechanisms implicated in limiting antitumor activity including KRAS nucleotide cycling and pathways that induce feedback reactivation and/or bypass KRAS dependence. These factors included activation of receptor tyrosine kinases (RTK), bypass of KRAS dependence, and genetic dysregulation of cell cycle. Combinations of MRTX849 with agents that target RTKs, mTOR, or cell cycle demonstrated enhanced response and marked tumor regression in several tumor models, including MRTX849-refractory models. SIGNIFICANCE :The discovery of MRTX849 provides a long-awaited opportunity to selectively target KRAS G12C in patients. The in-depth characterization of MRTX849 activity, elucidation of response and resistance mechanisms, and identifi cation of effective combinations provide new insight toward KRAS dependence and the rational development of this class of agents.

The progression of disease- and age-dependent skeletal muscle wasting results in part from a decline in the number and function of satellite cells, the direct cellular contributors to muscle repair1–10. However, little is known about the molecular effectors underlying satellite cell impairment and depletion. Elevated levels of inflammatory cytokines, including interleukin-6 (IL-6), are associated with both age-related and muscle-wasting conditions11–13. The levels of STAT3, a downstream effector of IL-6, are also elevated with muscle wasting14,15, and STAT3 has been implicated in the regulation of self-renewal and stem cell fate in several tissues16–19. Here we show that IL-6–activated Stat3 signaling regulates satellite cell behavior, promoting myogenic lineage progression through myogenic differentiation 1 (Myod1) regulation. Conditional ablation of Stat3 in Pax7-expressing satellite cells resulted in their increased expansion during regeneration, but compromised myogenic differentiation prevented the contribution of these cells to regenerating myofibers. In contrast, transient Stat3 inhibition promoted satellite cell expansion and enhanced tissue repair in both aged and dystrophic muscle. The effects of STAT3 inhibition were conserved in human myoblasts. The results of this study indicate that pharmacological manipulation of STAT3 activity can be used to counteract the functional exhaustion of satellite cells, thereby maintaining the endogenous regenerative response and ameliorating muscle-wasting diseases.

Fibro-adipogenic progenitors (FAPs) are typically activated in response to muscle injury, and establish functional interactions with inflammatory and muscle stem cells (MuSCs) to promote muscle repair. We found that denervation causes progressive accumulation of FAPs, without concomitant infiltration of macrophages and MuSC-mediated regeneration. Denervation-activated FAPs exhibited persistent STAT3 activation and secreted elevated levels of IL-6, which promoted muscle atrophy and fibrosis. FAPs with aberrant activation of STAT3-IL-6 signalling were also found in mouse models of spinal cord injury, spinal muscular atrophy, amyotrophic lateral sclerosis (ALS) and in muscles of ALS patients. Inactivation of STAT3-IL-6 signalling in FAPs effectively countered muscle atrophy and fibrosis in mouse models of acute denervation and ALS (SOD mice). Activation of pathogenic FAPs following loss of integrity of neuromuscular junctions further illustrates the functional versatility of FAPs in response to homeostatic perturbations and suggests their potential contribution to the pathogenesis of neuromuscular diseases.

Fibro-adipogenic progenitors (FAPs) are currently defined by their anatomical position, expression of non-specific membrane-associated proteins, and ability to adopt multiple lineages in vitro. Gene expression analysis at single-cell level reveals that FAPs undergo dynamic transitions through a spectrum of cell states that can be identified by differential expression levels of Tie2 and Vcam1. Different patterns of Vcam1-negative Tie2high or Tie2low and Tie2low/Vcam1-expressing FAPs are detected during neonatal myogenesis, response to acute injury and Duchenne Muscular Dystrophy (DMD). RNA sequencing analysis identified cell state-specific transcriptional profiles that predict functional interactions with satellite and inflammatory cells. In particular, Vcam1-expressing FAPs, which exhibit a pro-fibrotic expression profile, are transiently activated by acute injury in concomitance with the inflammatory response. Aberrant persistence of Vcam1-expressing FAPs is detected in DMD muscles or upon macrophage depletion, and is associated with muscle fibrosis, thereby revealing how disruption of inflammation-regulated FAPs dynamics leads to a pathogenic outcome.