Non-small cell lung cancer (NSCLC), a tumor type that encompasses adenocarcinoma and squamous cell carcinoma, constitutes 82 percent of lung cancer cases in the U.S. (CA Cancer J Clin 2022; https://doi.org/10.3322/caac.21708). Roughly 30 percent of patients with NSCLC have tumors that harbor mutations in the kelch-like ECH-associated protein 1 (KEAP1)-nuclear factor erythroid-2-related factor-2 (NRF2) pathway (Nat Genet 2016;48:607-616; doi: 10.1038/ng.3564); these mutations correlate with poor prognosis, as patients do not respond to chemotherapy, radiation, immunotherapy, or targeted therapies (Clin Cancer Res 2018;24(2):334-340; doi: 10.1158/1078-0432.CCR-17-1841).
Patients with mutations in this pathway either have loss-of-function mutations in KEAP1, which inactivate this gene's ability to promote degradation of the NRF2 protein, or alterations in NRF2, leading to increased NRF2 activity. The pathway makes tumors more aggressive and faster-growing, increasing the antioxidant capacity of tumor cells while also making the tumors resistant to standard-of-care and newer therapies (Cell 2019;178(2):316-329.e18; doi: 10.1016/j.cell.2019.06.003; Nat Med 2017;23(11):1362-1368; doi: 10.1038/nm.4407).
The frequency of KEAP1/NRF2 mutations has spurred numerous research efforts to identify druggable imbalances arising from high antioxidant production. From previous work, our group found that low glutamate levels are a hallmark of KEAP1-mutant tumors, which become very dependent on nutrients such as glutamine. Until recently, there was no good way to target glutamine or other glutamine-consuming enzymes in tumor cells. The discovery of DRP-104 (sirpiglenastat), a novel glutamine antagonist, appears to have changed that.
DRP-104 is a prodrug of DON (6-diazo-5-oxo-L-norleucine), a potent and well-characterized inhibitor of multiple glutamine-dependent enzymes. When administered systemically, DON showed dose-limiting gastrointestinal toxicity. DRP-104 was designed to be non-toxic and highly stable in the gastrointestinal tract and plasma and, therefore, is inactive in the circulation in most tissues. DRP-104 avoids toxicity because it is activated and converted to DON (via enzymatic cleavage) preferentially when it is present in the tumor microenvironment.
Preclinical data suggest that DRP-104 exerts anti-tumor activity and extends survival in KEAP1-mutant lung adenocarcinoma mouse models. By contrast, CB-839 (telaglenastat), a selective antagonist that inhibits only one glutamine-metabolizing enzyme, glutaminase, does not extend survival in these models. DRP-104 also demonstrates anti-tumor activity in KEAP1-mutant lung adenocarcinoma patient-derived xenograft (PDX) models and is superior to CB-839 in exerting anti-tumor activity in KEAP1-mutant squamous cell carcinoma PDX models. Lastly, we have seen very strong responses to DRP-104 in KRAS-G12C-KEAP1-mutant squamous cell carcinoma PDX models—an important finding because these mutations do not respond as well to G12C inhibition.
The encouraging preclinical activity of DRP-104 has prompted efforts to identify which glutamine-dependent reactions this agent inhibits. Presumably, inhibition of multiple enzymes would confer better anti-tumor activity. Preclinical data demonstrate that DRP-104 broadly inhibits all 10 glutamine-metabolizing enzymes. Additionally, metabolic analyses suggest DRP-104 suppresses multiple steps of nucleotide/purine synthesis. Administration of DRP-104 results in a dramatic increase in the substrates to PPAT and PFAS, suggesting a blockade of these glutamine-consuming enzymes.
That inhibitory action further suggests that nucleotide synthesis, which requires glutamine, is particularly inhibited in KEAP1-mutant cells, raising the question of whether metabolite supplementation can rescue DRP-104 sensitivity. In other words, if DRP-104 inhibits glutamine-dependent enzymes involved in nucleotide synthesis (e.g., PPAT, PFAS, GMPS), will the tumor cells grow again if we bypass these enzymes by substituting nucleotides and purines directly?
In fact, supplementation with a nucleotide mix—consisting of cytidine, hypoxanthine, uridine, thymidine, guanosine, and adenosine—appears to rescue DRP-104 sensitivity in a dose-dependent manner. Similarly, supplementation with hypoxanthine, which fuels purine nucleotide production, can rescue cell growth, at least at lower doses of DRP-104. Among glutamine antagonists, this inhibition of purine metabolism is unique to DRP-104.
Another distinguishing feature is that tricarboxylic acid cycle (TCA) carbon fuels do not rescue DRP-104 sensitivity, but they do rescue CB-839. Because the latter agent inhibits glutaminase and limits glutamate availability, tumor cells are unable to use glutamine and glutamate for the TCA cycle, which is the second stage of cellular respiration. Supplementation with TCA cycle substrates bypasses the need for glutamate, enabling unhindered cell growth even in the presence of CB-839 (eLife 2017;6:e28083; doi: 10.7554/eLife.28083), but not with DRP-104. Indeed, DRP-104, with its broader mechanism of action, seems to primarily affect nucleotide synthesis, an action that is detrimental to cancer cells.
In addition to its anti-tumor effect, DRP-104 may overcome checkpoint inhibitor resistance in KEAP1-mutant patients (Clin Cancer Res 2018;24(2):334-340; doi: 10.1158/1078-0432.CCR-17-1841; Science 2019;366(6468):1013-1021; doi: 10.1126/science.aav2588; Cancer Res 2021;81(13 Suppl):1563; doi: 10.1158/1538-7445.AM-2021-1563). It also appears to reprogram the tumor microenvironment by boosting anti-tumor immune responses. This is important because KEAP1 mutations are particularly refractory to immunotherapy. Combining DRP-104 with a PD-1 or PD-L1 inhibitor may therefore be beneficial; our laboratory is testing this strategy.
With its broad glutamine antagonism and particularly robust activity against nucleotide production, DRP-104 is potentially synergistic with the current standard of care in NSCLC, as well as with novel KRAS/G12C-selective inhibitors such as sotorasib and adagrasib. In addition to its demonstrated activity against highly aggressive subtypes of NSCLC, DRP-104 may have potential applications in other malignancies, such as head and neck cancer and castration-resistant prostate cancer, in which the KEAP1/NRF2 pathway can be altered. In the meantime, a first-in-human Phase I/IIa study of DRP-104 as a single agent in KEAP1/NRF2-mutant NSCLC is ongoing (NCT00471415), and is expected to yield additional valuable insights into the potential use of this novel agent.
THALES PAPAGIANNAKOPOULOS, PHD, is Associate Professor in the Department of Pathology at the New York University Grossman School of Medicine.