Celebrating the 10th Anniversary

A: (Yi Liu, Matthew R. Janes, Matthew P. Patricelli) Since this was the first time direct KRAS inhibitors were identified that convincingly showed low uM cellular activity though on-target mechanism, this study answered several questions related to KRAS as a drug target as well as questions related to KRAS biology.

Is KRAS druggable? This was the first convincing demonstration that small molecules that directly bind to KRAS-G12C mutant protein can suppress KRAS signaling and inhibit proliferation of cancer cells harboring KRAS G12C mutation. This work unambiguously demonstrated that the covalent approach of targeting KRAS G12C mutant pioneered by Kevan Shokat was feasible in cells. This finding unleashed a wave of drug development for targeting mutant KRAS G12C. With the first of KRAS G12C inhibitors being expected to be approved by FDA this year, this work paved the way for the final proof that KRAS is druggable.

Is the oncogenic mutant of KRAS constitutively active? The dogma in the field before this study was that oncogenic KRAS mutants are constitutively active. Using the identified KRAS G12C inhibitor and an elegant mass spectrometry-based assay established for this study, we convincingly demonstrated that KRAS G12C cycles fast between active “on” GTP bound state and inactive “off” GDP bound state.

Is targeting KRAS activator a valid approach of targeting oncogenic KRAS? It was a debate in the KRAS field whether targeting KRAS GTPases that facilitate active cycling of RAS such as SOS1, SHP2, including their partnering up-stream receptor tyrosine kinases would be a valid approach to suppress oncogenic KRAS signaling. This study demonstrated all of these approaches are possible and has since fueled several targeting strategies to block KRAS G12C cycling including combination strategies that target these upstream activators.

How dependent are cancer cells to direct chemical inhibition of mutant KRAS? Before this study, dependency could only be answered by RNAi or CRISPR-based genetic knockout studies. Using KRAS direct inhibitors, we confirmed several published genetic-based studies showing that only a fraction of KRAS-mutated cell lines were actually dependent on the oncogene to support growth and survival in 2-D assay format. However, when cultured under 3-D suspension we demonstrated all of the KRAS G12C tumor cell lines tested were sensitive to KRAS inhibition. This work strongly demonstrated that the KRAS signaling dependency would be underestimated using traditional adherent tissue culture formats,

A: (Yi Liu, Matthew R. Janes, Matthew P. Patricelli) What surprised us most was that the KRAS-G12C isoform was not "constitutively" bound to GTP in an active form as had been presumed for all mutant KRAS isoforms for more than 30 years based on early work in the field. Rather the protein cycled between active (GTP bound) and inactive (GDP bound) states and the level of the active form could be modulated by upstream signaling inputs. What excited us most was that KRAS was druggable using this approach initially developed by Kevan Shokat. This gave us a lot of confidence to continue working on this approach and eventually led to the identification of several clinical candidates by us and others.

A: (Yi Liu, Matthew R. Janes, Matthew P. Patricelli) The greatest impacts of the paper were its validation of the approach to covalently target KRAS-G12C, and the elucidation of the dependency of mutant KRAS on upstream signaling inputs. These findings played a key role in reshaping approaches to targeting KRAS mutant cancers. This work inspired the pursuit and development of several KRAS-G12C inhibitors currently in clinical trials, and multiple agents targeting KRAS through upstream signaling nodes such as SHP2 and SOS1. The finding that KRAS G12C mutant cycles between active and inactive states prompted others to study the cycling of other KRAS mutants and may form the basis of targeting other KRAS mutants using the same approach of targeting inactive KRAS.

A: (James G. Christensen) I believe this was the first published article to 1) demonstrate the breadth of a G12C inhibitor as monotherapy across in vivo models and provide genomic insight toward responder/non-responder populations (we believe that full interrogation of KRAS as an oncogenic driver is best evaluated in an in vivo context), 2) remains perhaps the most comprehensive article with regard to evaluation of G12C inhibitor mechanism-based combinations with multiple targeted therapies including mechanistic insight toward the ability of combinations to address feedback and bypass pathways (including one of the first with in vivo data) and 3) first published evidence of clinical activity for a G12C inhibitor indicating that KRAS mutations can function in an oncogenic driver context in human cancer.

A:(Jill Hallin) We were so thrilled to have found such a potent and selective KRAS G12C inhibitor, and because this molecule checked all the boxes for a drug-like candidate we were able to perform deeply comprehensive mechanistic analyses both in vitro and in vivo that opened our eyes to the breadth of efficacy of KRAS G12C inhibition. These data could then directly translate into possibility for clinical trial designs, both as single agent and with rationally considered combination partners.

A: (James G. Christensen) Our data on SHP2, mTOR, and CDK inhibitor combinations and mechanism preceded public domain data at the time we generated it so we very excited to see that we could further improve on the activity observed initially with MRTX849. We were very excited to learn of our first responding patients on our first clinical trial.

A: (Jill Hallin) The depth and breadth of responses in our in vivo CDX and PDX models as a monotherapy were extremely encouraging, and then building on what we learned from a genomic perspective on vulnerabilities in each model, we were able to make really informed decisions on high impact combination partners to improve the activity of MRTX849 when necessary. Seeing the data on patient benefit come out of our clinical trial and being able to include it in this manuscript was truly inspiring to our team.

A: (James G. Christensen) Since the publication is fairly recent, I would imagine the impact will become higher resolution in the future. My hope is that the publication’s legacy may be providing a deeper understanding of the mechanism of action for KRAS inhibitors, increasing understanding of feedback and bypass mediated resistance to this class of drugs, and sparking rational combination development to make a truly profound impact on patient lives.

A: (Jill Hallin) We hope that the impact and roadmap this paper provides to other researchers will help to deepen understanding of KRAS biology, and lead to comprehensive profiling of potential combination partners for KRAS G12C inhibitors in the clinic. We would be ecstatic if our paper can be a part of building understanding on how we can better treat patients, with a precision medicine approach that has potential to lead to better tolerated and more durable responses, and ultimately improved patient outcomes.