Traceable evidence for biochemistry

 Quick Analysis Plan Copied

 Long Analysis Plan

Critical premise check first

All supplied evidence establishes X6632 as a pan-ID protein inhibitor: it reduces ID1–ID4 at 10 µM/24 h in A375, HT144 and HUVECs X6632 pan-ID inhibitor paper, with the authors explicitly noting no detectable X6632–ID1 binding and an undefined mechanism. No supplied source reports X6632 interacting with USP1. Therefore the DARTS/CETSA plan below is a falsification-driven test, not a confirmation experiment. USP1 engagement controls exist: TNG348 (allosteric USP1 inhibitor, 98.5 nM biochemical IC50, 98.6 nM cellular ub-PCNA IC50) TNG348 USP1 inhibitor characterization and ML323 ML323 USP1 inhibition in K562 cells.

CETSA design (cell lysate / intact-cell thermal shift)

• Model: A375 and HUVEC (USP1 expressed; both X6632-responsive per cited data) plus HCC1954 (BRCA1-WT, USP1i-resistant comparator in TNG348 work).
• Treatment: intact cells, 1 h, X6632 at 0.3, 1, 3, 10, 30 µM (spanning and exceeding its reported IC50s); DMSO vehicle; TNG348 1 µM and ML323 10 µM as USP1-engagement positive controls; AGX51 20 µM as ID-targeting negative-modulation control.
• Heat gradient: 42–62 °C, 3 min; soluble fraction by 100,000 g spin; USP1, ID1, ID3, PCNA, and β-actin by immunoblot; quantify band intensity vs temperature, fit Boltzmann sigmoid, derive Tm (ΔTm ≥ 2 °C with p<0.05, n=3 biological replicates considered a hit).
• Orthogonal PD readout: parallel untreated-lysate Western for ub-PCNA/ub-FANCD2 accumulation (the TNG348-engagement marker) after 6–24 h X6632 — a functional, not physical, engagement test.

DARTS design (limited proteolysis)

• Lysates from ±1 h X6632-treated cells (3 and 10 µM); digest aliquots with pronase (0.5–5 µg/mg protein ratio titration) or trypsin; stop, immunoblot USP1 plus ID1/ID2 (the paper's own targets — a built-in specificity check since ID1 binding was undetectable).
• Protected USP1 fragment accumulation at 3–10 µM X6632 but not DMSO = engagement; simultaneous protection of ID1 would be an off-target/pan-protein-stabilization red flag.
• Compute fragment/intact ratio, two-way ANOVA (compound × protease), n=3.

Interpretation and blind spots

What would confirm: X6632-induced USP1 thermal shift + protease protection + ub-PCNA accumulation, ATP-depletion-ruled-out (add ATP post-lysis control), and detergent-insolubility ruled out (Triton X-100 test). What would refute: no shift, no protection, no ub-PCNA/ub-FANCD2 rise — consistent with the paper's own inability to detect X6632–ID1 binding, suggesting the SPR false-negative may instead reflect genuinely weak/transient binding or a non-USP1 mechanism (ID downregulation could be transcriptional). Known unknowns: X6632 PK, covalent vs reversible binding, USP1 abundance in these lines, and whether ID protein loss is upstream or downstream of USP1. Confidence: moderate — the assay logic is sound, but the underlying USP1 hypothesis rests on no direct evidence in the supplied records.