Review Papers Claim by Claim

Topline critical summary

The preprint Dual MYC and GSPT1 Protein Degrader for MYC-Driven Cancers (doi 10.1101/2025.04.24.650490) reports a first-in-class cereblon modulator GT19630 that the authors claim (1) binds CRBN and pulls down MYC and GSPT1, (2) induces proteasome-dependent degradation of MYC and GSPT1, (3) provokes integrated stress response (ISR) and inhibits OXPHOS/TCA flux, and (4) shows strong anti-tumor activity in multiple MYC-driven in vitro and in vivo models with a tentative therapeutic window in hematopoietic cells. These conclusions are supported by cell biology, proteomics/pull-downs, Seahorse and metabolomics, CRISPR/siRNA perturbations, reporter readthrough assays, and multiple mouse xenografts, but the study remains preclinical, derives from a single research group with industry links, and requires independent replication, structural ternary complex evidence, broader off-target profiling, and expanded non-rodent toxicology before translational claims are warranted GT19630 preprint

Detailed evidence-based critique and analysis of Dual MYC and GSPT1 Protein Degrader for MYC-Driven Cancers

Executive evidence summary

• Main claim GT19630 is described as a CRBN-engaging molecular glue that degrades both MYC and GSPT1, inducing ISR and metabolic collapse in MYC-addicted cancers and producing tumor regressions in multiple preclinical models GT19630 preprint discovery and mechanism
• Functional consequences The paper links dual degradation to integrated stress response (ATF4/ATF3 induction), stop codon readthrough (SCR) on engineered MYC reporters, OXPHOS inhibition, disrupted glutaminolysis and TCA cycle flux, and potent apoptosis across hematologic and some solid tumor lines GT19630 functional assays
• In vivo efficacy and tolerability GT19630 (and an oral analog GT19870) produced tumor regressions in multiple xenograft and PDX models (AML, Burkitt lymphoma, MM, SCLC, CRPC) and appeared tolerable in Crbn I391V humanized mice at tested doses, with moderate reversible leukopenia but no mortality in the studies shown GT19630 in vivo results

Strengths of the study

1. Multi-modal evidence: biochemical pull-downs, CRBN binding HTRF, CRISPR loss-of-function, proteasome inhibitor rescue, proteomics, reporter assays, RNAseq, metabolomics, and animal efficacy deliver converging mechanistic and phenotypic data consistent with the authors hypothesis Study methods summary
2. Biological plausibility: prior literature supports GSPT1 as a CRBN neo-substrate whose degradation triggers integrated stress response and lethality in AML models, making the approach mechanistically credible (see clinical/development precedent with CC-90009 and CELMoDs) CC-90009 GSPT1 CELMoD paper

Key limitations, blindspots and risks

• Lack of structural ternary complex data. The manuscript shows biochemical pull-downs and HTRF CRBN binding but does not present high-resolution structural evidence (X-ray/cryo-EM) of a CRBN:GT19630:MYC or CRBN:GT19630:GSPT1 ternary complex. Without ternary complex structural mapping the molecular glue claim is plausible but incomplete; specific residues mediating ternary engagement and whether MYC is a direct neosubstrate or co-purified via chromatin complexes remain unresolved Missing ternary structural evidence
• MYC mechanism ambiguity. MYC is a nuclear, intrinsically disordered transcription factor. The canonical route for CRBN neo-substrate recruitment involves a degron that becomes bound in a CRBN small-molecule pocket; how GT19630 recruits MYC (which lacks a previously described CRBN degron) is not mechanistically defined and could reflect indirect co-degradation through chromatin complexes, proteostasis collapse, or off-target effects. Direct genetic evidence that a defined MYC mutation/residue confers resistance (analogous to GSPT1 G575N with CC-90009) is lacking MYC T58I resistance observation
• Specificity and off-targets. The paper profiles some proteomic changes but does not provide a full degradome (global TMT/TMTpro proteomics comparing GT19630 and CC-90009 across concentrations/time) to define off-target neosubstrates (e.g., IKZF family, CK1alpha, GSPT2). Potential degradation of other essential proteins could contribute to toxicity; comprehensive proteome-wide substrate mapping is required Limited degradome data
• Model and translational gaps. Efficacy is shown in immunodeficient xenografts and humanized Crbn I391V mice; the immune consequences of MYC/GSPT1 dual degradation (immunogenicity, antigen presentation effects) and toxicity in non-rodent species are untested. The authors acknowledge need for further toxicology and species translation studies before clinical claims In vivo tolerability caveats
• Conflict of interest and reproducibility risks. The paper lists industry funding and author links to companies (Kintor, others). This does not invalidate findings but raises the need for independent replication and data sharing; the authors pledge to share materials and will deposit RNAseq upon acceptance which is appropriate but incomplete for immediate independent validation Funding and COI disclosures

Concrete experiments and data needed to strengthen the claims

1. High-resolution structural determination (cryo-EM or crystallography) of CRBN:GT19630:MYC and CRBN:GT19630:GSPT1 ternary complexes or at least mapping of MYC minimal degron required for recruitment and degradation to prove direct molecular glue mechanism Need structural ternary data
2. Global quantitative proteomics (TMT/TMTpro) timecourse at multiple concentrations in at least two human cell lines to define the degradome, Dmax/DC50 values for substrates, and on/off target profile compared to CC-90009 and other CELMoDs Need global degradome
3. Genetic selection/resistance mapping: generate resistant clones and sequence for CRBN, GSPT1, or MYC alterations (as was done for CC-90009 class with GSPT1 resistance mutations) to show causal degradation determinants CC-90009 precedent for resistance mapping
4. Cross-species toxicology and non-human primate CRBN binding assessment to evaluate safety window because murine CRBN differs from human CRBN and the Crbn I391V humanized mouse is a partial model; this is essential before translational claims Need non-rodent tox

Interpretive synthesis and balanced takeaways

The preprint presents a provocative and potentially high-impact strategy: direct reduction of MYC protein using a CRBN-engaging small molecule that also degrades GSPT1. The approach builds on validated biology that GSPT1 degradation activates the integrated stress response and can preferentially kill AML cells GSPT1 CELMoD precedent. The novel observation here is that MYC protein levels can also fall after GT19630 treatment and that dual targeting potentiates cell death and metabolic collapse. That said, the central mechanistic claim that MYC is a direct CRBN neo-substrate engaged by GT19630 requires stronger direct biochemical/structural proof and broader degradome mapping to exclude indirect explanations (e.g., accelerated MYC turnover secondary to ISR, translational readthrough, or co-purifying chromatin complexes). The preclinical efficacy is encouraging but must be tempered by COI disclosures, absence of independent replication so far, and the need for extensive toxicology and degradome profiling before clinical translation can be responsibly considered Preclinical promise with caveats.

Practical next steps for authors and the field

1. Provide ternary complex structural data or minimal degron mapping for MYC showing direct recruitment to CRBN in a GT19630-dependent manner.
2. Share complete TMT degradome datasets and raw RNAseq/metabolomics for independent analysis and meta-analyses.
3. Perform resistance selection and sequencing to map causal degron/resistance mutations for MYC and GSPT1.
4. Expand in vivo safety to a non-rodent species and profile immune consequences in immunocompetent models or syngeneic systems if possible.
5. Compare GT19630 to existing GSPT1 CELMoDs (CC-90009 family) in head-to-head degradome and toxicity assays to demonstrate a clear therapeutic index advantage.

Confidence and conclusion

Confidence in the experimental observations reported (decrease in MYC and GSPT1 protein after GT19630 treatment, ISR induction, OXPHOS/TCA effects, and in vivo tumor regressions in the presented models) is moderate because the data are multi-modal and internally consistent, but major mechanistic gaps (direct recruitment proof) and reproducibility/COI considerations lower translational confidence until independent validation and additional experiments are completed Authors conclusion and caveats

Author reviews