Abstract: The Kirsten rat sarcoma viral oncogene homolog (KRAS) is a frequently mutated oncogene responsible for a significant portion of cancer diagnoses. While recent breakthroughs have led to the development of covalent inhibitors targeting the KRAS G12C mutation, the emergence of acquired resistance through secondary mutations has necessitated the development of broader therapeutic strategies. BI-2865 has emerged as a highly potent, non-covalent pan-KRAS inhibitor designed to target multiple KRAS mutations and potentially address the challenge of acquired resistance. This review explores the pharmacological activity, molecular mechanism of action, and structure-activity relationship of BI-2865. Furthermore, it highlights current limitations, such as the concurrent inhibition of wild-type KRAS and the potential for secondary switch-II pocket mutations, while providing future perspectives on combination therapies to overcome resistance in KRAS-driven cancers.
1. Introduction
The Kirsten rat sarcoma viral oncogene homolog (KRAS) is the most frequently mutated oncogene in human cancers, affecting approximately 30% of cancer diagnoses [1]. For decades, KRAS was considered "undruggable" due to the lack of pharmacological binding pockets. However, recent breakthroughs led to the accelerated FDA approval of the first covalent inhibitors targeting the KRAS G12C mutation, such as Sotorasib and Adagrasib [1]. Despite this unprecedented clinical success, acquired resistance mechanisms have rapidly emerged, presenting a continuously moving target for drug development. Cancer cells evade KRAS inhibition through various routes, notably via secondary point mutations within the drug-binding pocket (e.g., Y96, R68, H95) and KRAS gene amplification [1]. To combat these resistance mechanisms and expand treatment options beyond the G12C subpopulation, researchers have developed novel pan-KRAS inhibitors. Among these, BI-2865, developed by Boehringer Ingelheim, represents a significant breakthrough as a non-covalent pan-KRAS inhibitor capable of targeting a wide array of KRAS mutants [1].
2. Pharmacological Activity
BI-2865 exhibits broad and potent pharmacological activity against multiple KRAS variants. In preclinical studies, it has been shown to successfully reduce tumor proliferation in both cell lines and mouse models [1]. The compound is highly selective for KRAS, demonstrating approximately 2-3 orders of magnitude greater selectivity for KRAS over other RAS isoforms, namely HRAS and NRAS [1]. BI-2865 effectively inhibits many common oncogenic mutations, displaying strong binding affinities (Kd) across various states: KRAS WT (6.9 nM), KRAS G12C (4.5 nM), KRAS G12D (32 nM), KRAS G12V (26 nM), and KRAS G13D (4.3 nM) [1]. It is broadly active against G12, G13, and Q61H mutations, though it notably lacks efficacy against the G12R mutation and other Q61 mutations [1].
3. Molecular Mechanism of Action
Unlike the first-generation G12C-specific drugs that rely on covalent bonds with a reactive cysteine, BI-2865 functions as a non-covalent inhibitor [1]. It is specifically designed to target the inactive (GDP-bound) form of the KRAS protein [1]. BI-2865 binds to the allosteric switch-II pocket of KRAS, a critical region involved in the dynamic conformational changes required for RAS signaling [1]. By occupying this pocket, BI-2865 stabilizes the inactive state of the protein, thereby preventing the activation of downstream effector pathways that drive cancer cell proliferation.
4. Structure-Activity Relationship (SAR)
The structural design of BI-2865 was based on G12C pharmacophores and is accompanied by atomic-resolution crystal structures [1]. Structural analysis reveals that BI-2865 confers its high selectivity for KRAS over HRAS and NRAS primarily around the H95 residue [1]. H95 is one of the few amino acids within the RAS G-domain that differs among the various RAS isoforms, making it a critical anchor point for isoform-specific drug design [1].
Furthermore, the binding mode of BI-2865 in the switch-II pocket is structurally very similar to that of Adagrasib and the G12D-specific inhibitor MRTX-1133 [1]. Four specific residues in the switch-II pocket—R68, H95, Y96, and Q99—make crucial contacts with BI-2865 [1]. Because these residues are integral to stabilizing the drug-protein interaction, alterations at these sites directly impact the binding affinity and efficacy of the inhibitor.
5. Current Limitations
Despite its broad efficacy, the use of BI-2865 presents several limitations. First, as a pan-KRAS inhibitor, BI-2865 also inactivates wild-type (WT) KRAS [1]. Currently, there is no clinical data demonstrating the physiological consequences of blocking WT KRAS, raising concerns about potential toxicities and how well cancer patients will tolerate this pan-KRAS inhibition [1]. Second, BI-2865 is not universally effective against all KRAS mutations; it fails to inhibit the G12R mutation and certain Q61 mutations [1].
Finally, acquired resistance remains a significant threat. Because BI-2865 shares a very similar binding mode in the switch-II pocket with Adagrasib and MRTX-1133, secondary mutations that confer resistance to G12C inhibitors (such as mutations at R68, H95, Y96, and Q99) could potentially arise during BI-2865 treatment [1]. These secondary mutations impart selective pressure on ligand stability and can disrupt the crucial contacts required for the non-covalent binding of BI-2865.
6. Future Perspectives
The development of BI-2865 marks a significant step forward in the direct drugging of RAS, but future research must address its limitations. Clinical trials and long-term studies are urgently needed to assess the effectiveness of BI-2865 against cancer cell resistance and to evaluate the safety profile regarding WT KRAS inhibition [1]. It is hypothesized that the potential toxicity from WT KRAS inhibition might be compensated by the activity of NRAS and HRAS isoforms, but this requires clinical validation [1].
To circumvent acquired resistance mechanisms, future therapeutic strategies will likely rely on combination therapies. Combining KRAS inhibitors like BI-2865 with immunotherapies—such as bispecific T-cell engagers (BiTEs) or anti-PD-1 treatments—shows promise in altering the immunosuppressive tumor microenvironment, though toxicities must be carefully managed [1]. Additionally, targeting alternative pathways, such as the inositol-requiring enzyme 1a (IRE1a) branch of the unfolded protein response, alongside KRAS inhibition, may provide synergistic effects to overcome resistance and improve patient outcomes [1].
