Test Your Hypothesis

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 Long Analysis Plan

Evidence: ABCG2 as a transport gatekeeper

The strongest quantitative support for an ABCG2 efflux gate comes from isogenic pairs: H460/MX20 cells show IC50 shifts of ~435-fold for mitoxantrone, ~135-fold for SN-38, and ~8-fold for doxorubicin versus ABCG2-low H460, while cisplatin (~1.0-fold) is unaffected; co-treatment with the selective reversible inhibitor CCTA-1523 (5 μM) collapses these resistance ratios to ~7.5, ~2.4, and ~2.9 respectively CCTA-1523 ABCG2 inhibitor IC50 data. Axitinib similarly reverses ABCG2-mediated resistance to topotecan/mitoxantrone in SP-enriched stem-like cells in vitro and in vivo without changing ABCG2 expression Axitinib reverses ABCG2-mediated MDR.

Directionality is not always efflux: the critical blindspot

The ruxolitinib record shows ABCG2 can facilitate import and potentiate cytotoxicity: KO143 pre-treatment increases viability of ABCG2+ cells, and LC-MS intracellular ruxolitinib was 1.33-fold higher in ABCG2-expressing cells (3/5 repeats, SD ~0.24) — inconsistent directionality that complicates any assumption ABCG2 uniformly protects cells ABCG2 import of ruxolitinib potentiates cytotoxicity. This means the "gatekeeper" direction (efflux → CDK-inhibitor resistance vs import → sensitization) must be measured per compound, not assumed.

Biomarker stratification rationale from cell-state biology

ABCG2+ and ABCG2− tumor cells are similarly tumorigenic but occupy different positions in a dynamic progenitor–stem hierarchy, with interconversion and higher stemness gene expression (Notch-1, β-catenin, SMO, Oct-4) in ABCG2− cells ABCG2+/ABCG2− cancer cell hierarchy and side population. A common loss-of-function allele (Q141K, rs2231142; 54% reduced transport in oocytes) provides a human genetic stratification variable that could modulate intracellular drug exposure ABCG2 Q141K loss-of-function polymorphism.

What is missing for CDK inhibitors in TNBC

No supplied record tests whether any CDK inhibitor (e.g., transcriptional CDK7/9/12 inhibitors) is an ABCG2 substrate, nor any TNBC-specific IC50 data. The plan therefore requires: (1) parental vs ABCG2-overexpressing/knockout TNBC IC50 panels per CDK inhibitor; (2) intracellular drug quantification (LC-MS) to establish transport direction; (3) stratified analysis by ABCG2 expression/SP fraction and rs2231142 genotype; (4) per-drug effect estimation (CDK inhibitors are not interchangeable — species-specific substrate profiles differ even between canine and feline ABCG2 Species-specific ABCG2 substrate profiles). Confidence in the gatekeeper framing for classic ABCG2 substrates is moderate-to-strong; for CDK inhibitors specifically, it is untested.