KRAS G12D is an important driver mutation in solid tumors such as pancreatic, colorectal, and lung cancers. The emergence of molecules such as MRTX1133 has demonstrated that KRAS G12D can be directly targeted pharmacologically, whereas targeted protein degradation technologies such as PROTACs have further advanced the mode of action from “occupying and inhibiting KRAS” to “recruiting an E3 ligase and eliminating the KRAS protein.”

Keywords: KRAS G12D; MRTX1133; RAS; PROTAC; targeted protein degradation; ASP3082; VHL; CRBN; ubiquitin–proteasome system

1 Molecular Basis of KRAS G12D

1.1 The GDP/GTP Molecular Switch of KRAS

(1) GDP- and GTP-Bound States

KRAS belongs to the RAS family of small GTPases and switches between a GDP-bound low-activity state and a GTP-bound active state. GTP binding induces conformational changes in the Switch I and Switch II regions, enabling KRAS to recruit effector proteins such as RAF, PI3K, and RalGDS.

(2) Regulation by GEFs and GAPs

Guanine nucleotide exchange factors (GEFs) such as SOS1 promote GDP release and GTP loading, shifting KRAS into its active state. GTPase-activating proteins (GAPs) such as NF1 promote GTP hydrolysis and return KRAS to the GDP-bound state.

(3) The G12D Mutation

KRAS G12D results from the substitution of glycine at position 12 with aspartate. This position is located near regions involved in nucleotide binding and regulation of GTP hydrolysis. The mutation impairs GAP-stimulated GTP hydrolysis, favoring maintenance of KRAS in an active state and persistent activation of signaling pathways such as RAF–MEK–ERK and PI3K–AKT.

1.2 Why KRAS G12D Was Historically Difficult to Target Directly

(1) Absence of an Exploitable Mutant Cysteine

Drug development for KRAS G12C exploits the mutant Cys12 residue to establish covalent binding. G12D instead introduces a negatively charged Asp12 residue and cannot directly replicate the covalent inhibitory strategy used for G12C.

(2) Difficulty Competing at the Nucleotide-Binding Site

KRAS has high affinity for GDP and GTP, and intracellular guanine nucleotide concentrations are also high. Direct competition at the GDP/GTP-binding site therefore makes it difficult to achieve both sufficient affinity and selectivity.

(3) Highly Dynamic Switch Regions

GDP/GTP cycling is accompanied by continuous conformational changes in the Switch I and Switch II regions. The same KRAS mutation may produce distinct drug-binding environments in different nucleotide states, requiring drug design to account for both mutation selectivity and conformational selectivity.

1.3 Downstream Signaling Driven by KRAS G12D

(1) RAF–MEK–ERK

Activated KRAS recruits RAF family proteins and initiates the RAF–MEK–ERK cascade. ERK subsequently regulates transcriptional programs involving MYC, ELK1, and other factors, promoting cell-cycle entry, proliferation, and survival.

(2) PI3K–AKT

KRAS can also functionally engage PI3K, increasing PIP3 production and promoting AKT-related signaling involved in metabolic adaptation, protein synthesis, and resistance to apoptosis.

(3) A Signaling Network Rather Than a Single Pathway

KRAS G12D tumors do not depend exclusively on a single MAPK output. RTKs, SHP2, SOS1, wild-type RAS, and PI3K–AKT may all participate in feedback compensation, creating a basis for resistance to both direct inhibition and targeted degradation.

2 How Direct Inhibition of KRAS G12D Is Achieved

2.1 MRTX1133 Established a Noncovalent Direct-Inhibition Paradigm

(1) Recognition of a Switch-II-Related Pocket

MRTX1133 is a representative direct inhibitor of KRAS G12D. It binds the Switch-II-related pocket of KRAS G12D noncovalently with high affinity and exploits Asp12 and its surrounding structural environment to achieve mutation selectivity.

(2) Restriction of Nucleotide Exchange

After binding KRAS G12D, MRTX1133 reduces SOS1-mediated nucleotide exchange, decreasing the efficiency with which KRAS returns to its active GTP-bound state.

(3) Blockade of Effector-Protein Signaling

Occupation of KRAS G12D restricts its functional interactions with effector proteins such as RAF, subsequently reducing MEK and ERK signaling output.

2.2 Expansion from Single-Conformation Inhibition to Multistate KRAS Control

(1) The GDP-Bound State Is Not the Only Pharmacological Entry Point

The G12D mutation increases the proportion of KRAS in its active state. If a drug depends strongly on the GDP-bound state, SOS1 activity and the rate of nucleotide cycling may alter the proportion of KRAS available for drug binding.

(2) The RAS(ON) Strategy

Inhibitory strategies targeting active GTP-bound RAS extend pharmacological intervention to the RAS(ON) state. The ultimate objective of these drugs remains the restriction of functional interactions between KRAS and downstream effector proteins rather than reduction of KRAS protein abundance.

(3) Inhibition and Degradation Must Be Distinguished

Even if a direct inhibitor depends on the formation of a multicomponent complex, it cannot be defined as a targeted protein degrader unless E3 ligase-dependent ubiquitination and removal of the KRAS protein occur.

2.3 Major Limitations of Direct Inhibition

(1) Dependence on Target Occupancy

Conventional inhibitors primarily follow an occupancy-driven pharmacological model. Stable signal suppression requires sustained drug occupancy of a sufficient proportion of KRAS molecules.

(2) RTK–SHP2–SOS1 Feedback

ERK inhibition can relieve existing negative feedback, allowing RTKs such as EGFR to become reactivated and increase RAS activation input through SHP2 and SOS1.

(3) Compensatory Bypass Signaling

Following MAPK inhibition, parallel survival pathways such as PI3K–AKT may become enhanced. The initial reduction in p-ERK may therefore fail to translate into sustained tumor suppression.

(4) Binding-Site-Mediated Resistance

Direct inhibition depends strongly on stable interactions between KRAS and the drug. Secondary alterations in the Switch regions or adjacent residues may reduce drug affinity and confer resistance.

3 Why Advance from Direct Inhibition to Targeted Degradation?

3.1 From Occupancy-Driven to Event-Driven Pharmacology

Direct inhibition can be summarized as follows:

KRAS G12D + inhibitor → functional blockade of KRAS

PROTAC-mediated targeted degradation can be represented as follows:

KRAS G12D + degrader + E3 ligase → ternary complex → KRAS ubiquitination → proteasomal degradation

The principal difference is that direct inhibition mainly alters the functional state of KRAS, whereas degradation directly reduces intracellular KRAS protein abundance.

3.2 Three Fundamental Modules of a KRAS G12D PROTAC

(1) KRAS-Binding Moiety

The KRAS-binding moiety recognizes the G12D mutant protein. The development of high-affinity G12D-binding scaffolds such as MRTX1133 provided usable warheads for subsequent PROTAC design.

(2) E3 Ligase-Binding Moiety

VHL and CRBN are commonly recruited E3 systems in targeted protein degradation research. The E3 ligand determines which ligase is recruited and can also alter ternary-complex formation and degradation capacity in different tissue contexts.

(3) Linker

The linker connects the KRAS ligand to the E3 ligand. Linker length, flexibility, polarity, and attachment position collectively determine the spatial orientation between KRAS and the E3 ligase and can markedly affect cellular permeability.

3.3 The Ternary Complex Is Central to Degradation Efficiency

(1) Binary Affinity Is Insufficient to Predict Degradation

Connecting a high-affinity KRAS ligand to an E3 ligand does not necessarily produce efficient degradation. If the spatial arrangement between KRAS and the E3 ligase is unfavorable for ubiquitin transfer, measurable degradation may not occur even when both ends bind their respective targets.

(2) Cooperativity

KRAS, the PROTAC, and the E3 ligase can form new protein–protein interfaces. Positive cooperativity increases ternary-complex stability and the probability of productive ubiquitination.

(3) Spatial Accessibility of Lysine Residues

Ubiquitin must be transferred to lysine residues on the surface of KRAS. A productive ternary complex must position suitable lysine residues within the E2–E3 catalytic space.

3.4 Pharmacological Changes Introduced by Targeted Degradation

(1) Reduction of Total KRAS Protein

Direct inhibitors generally do not substantially alter total KRAS protein, whereas the central pharmacodynamic endpoint of a degrader is a reduction in KRAS abundance.

(2) Simultaneous Removal of Multiple Functions

KRAS possesses GTPase activity and serves as a binding platform for effector proteins such as RAF and PI3K. Protein degradation can simultaneously reduce these functions.

(3) Pharmacological Effects Can Outlast Transient Occupancy

After KRAS has been eliminated, new protein synthesis is required to restore its abundance. KRAS recovery after drug withdrawal may therefore lag behind extracellular drug clearance.

(4) Degradation Does Not Automatically Eliminate All Resistance

PROTACs still require their KRAS-binding moieties to recognize the target protein. Secondary KRAS mutations that disrupt warhead binding may therefore confer resistance to degradation.

4 How KRAS G12D-Targeted Degradation Is Established

4.1 Converting a Direct Inhibitor into a Degradation Warhead

(1) Identifying a Solvent-Exposed Position

When converting a direct inhibitor into a PROTAC, the linker must be installed at a position that does not substantially disrupt KRAS binding. An inappropriate attachment site may directly compromise the original affinity.

(2) Connecting an E3 Ligand

Connecting a KRAS warhead to a VHL or CRBN ligand converts a simple KRAS-occupying molecule into a bifunctional compound capable of simultaneously binding the target protein and an E3 ligase.

(3) Re-establishing Structure–Activity Relationships

A PROTAC cannot directly inherit the structure–activity relationship of its parent inhibitor. KRAS binding, E3 binding, ternary-complex stability, cellular permeability, DC50, and Dmax must all be reassessed.

4.2 VHL-Based KRAS G12D Degradation

(1) VHL Recruitment

A VHL-based PROTAC links a KRAS G12D ligand to a VHL ligand, bringing KRAS into proximity with the CRL2^VHL ubiquitin ligase complex.

(2) KRAS Ubiquitination

Following formation of a productive ternary complex, lysine residues on the KRAS surface are modified with ubiquitin, followed by the formation of polyubiquitin chains suitable for proteasomal recognition.

(3) Proteasomal Elimination

Polyubiquitinated KRAS is recognized and degraded by the 26S proteasome, reducing total KRAS G12D abundance and producing sustained suppression of downstream MAPK signaling.

4.3 The Clinical-Stage Degradation Strategy Represented by Setidegrasib (ASP3082)

(1) The KRAS G12D–E3 Ternary Complex

ASP3082 is an important representative of KRAS G12D-targeted degradation. Its pharmacological core is not simply increased KRAS-binding affinity but the establishment of a stable and productive KRAS G12D–degrader–VHL ternary complex.

(2) From Affinity Optimization to Complex Optimization

Conventional small-molecule structure–activity relationships focus primarily on KD, IC50, and cellular activity. Degrader development must additionally consider ternary-complex formation, cooperativity, ubiquitination efficiency, DC50, Dmax, and degradation duration.

(3) From Proof of Concept to Clinical Development

ASP3082, now known as setidegrasib, has advanced KRAS G12D-targeted degradation from clinical proof of concept into Phase III registrational development. Phase I studies have demonstrated KRAS G12D target degradation and antitumor activity in patients, further establishing that mutant KRAS can be actively eliminated as an engineered substrate of the ubiquitin–proteasome system.

4.4 E3 Ligase Selection Beyond VHL

(1) CRBN

CRBN is another established E3 recruitment system for PROTAC development. CRBN ligands based on IMiD scaffolds provide established chemical modification sites for constructing KRAS degraders with different linker lengths.

(2) Different E3 Ligases Are Not Directly Interchangeable

Transferring the same KRAS warhead from a VHL-based system to a CRBN-based system markedly alters ternary-complex geometry. Degradation may improve or disappear entirely.

(3) Tissue Expression Must Be Considered

The expression of different E3 ligases varies between tumors and normal tissues. The selectivity of future KRAS G12D degraders may therefore depend not only on the KRAS-binding moiety but also on the tissue distribution of the recruited E3 ligase.

5 Resistance and Combination Interventions Following KRAS G12D Targeting

5.1 RTK–SHP2–SOS1 Feedback

(1) Enhanced RTK Feedback

Inhibition of KRAS–ERK signaling relieves ERK-associated negative feedback, allowing receptor tyrosine kinases such as EGFR to increase upstream signaling input.

(2) SHP2 Reconnects RTKs to RAS

SHP2 participates in signal transmission from multiple RTKs to RAS. Increased SHP2 activity can enhance GRB2–SOS1-related input and promote reactivation of residual RAS.

(3) SOS1 Increases GTP Loading

SOS1 promotes the transition of RAS from its GDP-bound state to its GTP-bound state. SOS1 inhibition can reduce RAS-GTP formation and attenuate upstream feedback arising after direct inhibition or degradation.

5.2 MAPK and PI3K–AKT Compensation

(1) ERK Rebound

An early reduction in p-ERK does not indicate long-term MAPK pathway shutdown. RTK feedback, RAS paralogs, or changes at the RAF–MEK nodes may reactivate ERK.

(2) AKT Compensation

Some cells maintain survival by increasing PI3K–AKT activity, producing drug tolerance even when MAPK signaling remains relatively low.

(3) Combination Strategies Require Mechanistic Evidence

Before combining KRAS G12D-targeted interventions with MEK, ERK, SHP2, SOS1, or PI3K inhibitors, the corresponding feedback or bypass pathway should first be shown to increase following KRAS G12D intervention.

5.3 Degrader-Specific Resistance

(1) Alterations in E3 Ligases

Reduced expression of VHL, CRBN, or components of their respective complexes can decrease the efficiency with which ternary-complex formation is converted into productive ubiquitination.

(2) Alterations at the Ternary-Complex Interface

Alterations on the surface of KRAS or the E3 ligase may preserve binary binding while preventing formation of a stable and productive ternary complex.

(3) Reduced Ubiquitination Efficiency

Changes in the spatial accessibility of ubiquitin-accepting lysine residues on the KRAS surface can result in normal binding but insufficient degradation.

(4) Protein Resynthesis

Following KRAS degradation, compensatory transcription and translation may accelerate protein recovery. Both degradation and recovery rates must be incorporated into pharmacodynamic evaluation.

6 Experimental Distinction Between Direct Inhibition and Targeted Degradation

6.1 Confirming the KRAS G12D Model

(1) Mutational Background

Cells, organoids, or tumor tissues should first be confirmed to carry KRAS G12D. Depending on the study objective, KRAS WT, G12C, G12V, G13D, and other controls should be included.

(2) Total KRAS Protein

Total KRAS protein reflects protein abundance and is a central indicator for distinguishing direct inhibition from targeted degradation.

(3) RAS-GTP

RAS-GTP reflects the activation state of RAS. A reduction in RAS-GTP demonstrates only that activation has been inhibited and does not directly establish KRAS degradation.

6.2 Evidence Chain for Direct Inhibition

(1) Target Binding

SPR, ITC, CETSA, or intracellular target-engagement systems can be used to evaluate direct interactions between a candidate molecule and KRAS G12D.

(2) Functional Blockade

RAS-GTP, KRAS–RAF interactions, p-MEK, and p-ERK can be measured to establish the relationship between KRAS binding and downstream signal suppression.

(3) Maintenance of Total Protein

If total KRAS protein remains largely stable while RAS-GTP and p-ERK decrease markedly, the results are more consistent with direct functional inhibition.

6.3 Both DC50 and Dmax Must Be Evaluated for Degradation

(1) DC50

DC50 represents the degrader concentration required to achieve 50% of the maximum degradation effect and differs conceptually from the IC50 of a conventional inhibitor.

(2) Dmax

Dmax represents the maximum proportion of protein degradation achieved under the experimental conditions. A low DC50 does not necessarily indicate complete KRAS elimination.

(3) Time-Course Kinetics

The time points at which KRAS begins to decline, reaches maximum degradation, and subsequently recovers should be recorded rather than examining only a single endpoint.

(4) Washout

Continued monitoring of KRAS and p-ERK after degrader removal can reveal the rate of target-protein resynthesis and the duration of the pharmacological effect generated by degradation.

6.4 Attribution to the Ubiquitin–Proteasome Mechanism

(1) KRAS mRNA

KRAS mRNA should be measured concurrently when KRAS protein decreases. Rapid protein loss with largely stable mRNA provides stronger support for post-transcriptional regulation of protein abundance.

(2) Ubiquitination

Detection of increased ubiquitination after KRAS immunoprecipitation can connect ternary-complex formation with subsequent processing of the target protein.

(3) Proteasomal Rescue

Restoration of KRAS protein by proteasome inhibitors such as MG-132 supports the involvement of the proteasome in the degradation process.

(4) E3 Ligase Dependence

Competition or genetic knockdown of VHL or CRBN and negative-control molecules lacking E3-binding capacity can determine whether degradation depends on the corresponding E3 ligase.

(5) Cullin–RING Dependence

VHL and CRBN belong to Cullin–RING E3 systems. Inhibition of NEDD8 activation can further determine whether a Cullin E3 ligase participates in degradation.

6.5 Mutation Selectivity and Off-Target Evaluation

(1) Different KRAS Mutations

At minimum, G12D should be compared with WT. G12C, G12V, G13D, Q61H, and other mutant proteins or cellular backgrounds can also be included.

(2) RAS Paralog Proteins

HRAS and NRAS should not exhibit marked concurrent reductions under highly selective G12D degradation conditions. Otherwise, insufficient selectivity of the warhead or ternary complex should be considered.

(3) Proteomics

Proteomic analysis can determine whether a degrader causes unintended loss of other proteins and is an important complement to high-quality PROTAC selectivity studies.

6.6 Functional Endpoints

(1) MAPK Output

p-MEK and p-ERK reflect the strength of KRAS downstream signaling but must be analyzed together with changes in total KRAS protein.

(2) Cellular Proliferation and Survival

Cell viability, colony formation, and long-term proliferation can be used to evaluate the functional consequences of target intervention.

(3) In Vivo Pharmacodynamics

Animal studies should simultaneously record tumor inhibition, the depth of KRAS degradation in tumor tissue, and changes in p-ERK rather than inferring degradation solely from tumor volume.

7 Products

7.1 KRAS G12D Target Proteins, Genetic Tools, and Detection Products

7.2 Products for Direct KRAS G12D Inhibition Research

7.3 KRAS G12D and Comparative Targeted Degradation Products

7.4 E3 Ligase and Ubiquitin–Proteasome Research Products

7.5 SOS1–KRAS and Pan-KRAS Research Products

KRAS G12D drug development has expanded from direct occupancy-based inhibition of the mutant protein to E3 ligase-mediated protein elimination. Targeted degradation adds protein abundance as a new pharmacological control dimension, but its advantages still depend on ternary-complex formation, degradation selectivity, feedback-mediated resistance, and the therapeutic window.

References

[1] Mondal K, Posa MK, Shenoy RP, Roychoudhury S. KRAS Mutation Subtypes and Their Association with Other Driver Mutations in Oncogenic Pathways. Cells. 2024;13(14):1221. doi: 10.3390/cells13141221.

[2] Simanshu DK, Nissley DV, McCormick F. RAS Proteins and Their Regulators in Human Disease. Cell. 2017;170(1):17-33.

[3] Wang X, et al. Identification of MRTX1133, a Noncovalent, Potent, and Selective KRASG12D Inhibitor. J Med Chem. 2022;65(4):3123-3133.

[4] Hallin J, et al. Anti-tumor efficacy of a potent and selective non-covalent KRASG12D inhibitor. Nat Med. 2022;28(10):2171-2182.

[5] Békés M, Langley DR, Crews CM. PROTAC targeted protein degraders: the past is prologue. Nat Rev Drug Discov. 2022;21(3):181-200.

For more related articles, please see below:

[1] KRAS Inhibitors — The "Undruggable Target"? A Thing of the Past

[2] 2025 KRAS Targeted Drug Progress

[3] Traditional Chinese Medicine Molecule RA-V: A Potential Synergistic Factor in KRAS G12C Inhibition