Reviews/Targeted Therapy Prostate Cancer/Androgen receptor inhibitor sequence cross-resistance
Arpi · PC-ARPI-03 · v2
When one of these two hormone-blocking prostate cancer drugs stops working, switching to the other usually helps only modestly, and the corpus shows this repeatedly. A prospective study found that enzalutamide given after abiraterone produced a prostate-specific antigen response in only about a quarter of men [2], a randomised study found only a few months of control when enzalutamide followed abiraterone [11], and adding one drug to the other did not rescue resistant disease [1]. Laboratory work and a genomic tracking study suggest the shared androgen-receptor pathway is involved [14][15][17]. The panel accepted the clinical observation of reduced second-drug efficacy as well supported, but was more cautious about the claim that it is specifically caused by shared androgen-receptor changes, because one study found androgen-receptor markers were not independently predictive once other factors were considered [9]. That mix of strong clinical signal and partly indirect mechanism is why the panel stopped short of its highest tier.
The corpus consistently demonstrates that a second androgen-receptor pathway inhibitor delivers only modest activity after the first: enzalutamide after abiraterone gave a 27% PSA response and 8.1 mo radiographic PFS [2], and 3.8 mo PSA-PFS after abiraterone [11]; combining abiraterone with enzalutamide after enzalutamide progression showed no benefit (PFS 5.7 vs 5.6 mo, HR 0.83) [1]; and sequence order was associated with different combined PFS [3]. On mechanism, ctDNA profiling during sequential abiraterone and enzalutamide showed the androgen-receptor genotype continues to evolve, with an average 50% increase in AR copy number, and attributes acquired cross-resistance to AR [15]. Preclinical work shows enzalutamide-resistant cells are cross-resistant to abiraterone via AR-V7 [14] and across antiandrogens via the AKR1C3/AR-V7 axis [17]. Against a clean AR attribution, a liquid-biopsy cohort found no single AR perturbation remained prognostic after multivariable analysis, with tumour burden and TP53 outperforming AR biomarkers [9]. The contested point: The division is over the causal clause 'mediated by shared AR pathway alterations' rather than the clinical phenomenon. Mechanistic support is either preclinical [14][17] or associative genomic evolution [15], while [9] shows AR perturbations are not independently prognostic once tumour burden and TP53 are accounted for, leaving the mechanism as supported but not cleanly isolated.
All 19 papers behind this claim with their DOIs and what each does to it, the panel’s arbitration, the evidence gap map and the open research questions. Free, and sent to your inbox.
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One reviewer's only job is to attack the finding. The challenges below were weighed before the grade was set.
The causal phrase 'mediated by shared AR pathway alterations' overreaches: [9] found no single AR perturbation remained associated with prognosis after multivariable analysis, and TP53/tumour burden outperformed AR biomarkers — cross-resistance appears multifactorial, not cleanly AR-mediated.
The mechanistic AR-V7/AKR1C3 evidence ([14][17]) is cell-line and xenograft work; causal mediation in patients is not demonstrated, so the mechanism limb rests on indirect/associative clinical data ([15]) plus preclinical inference.
Where it remains uncertain. A liquid-biopsy cohort found no single AR perturbation remained prognostic after multivariable analysis, with TP53 and tumour burden outperforming AR biomarkers [9], and the mechanistic AR-V7/AKR1C3 evidence is cell-line and xenograft work [14][17].
Aleatory
Cross-resistance is not absolute; some patients still benefited from the second agent [2], so individual outcomes vary around the modest average.
Epistemic
Whether shared AR alterations are the cause rests on preclinical models [14][17] and associative genomic evolution [15]; [9] shows AR markers are not independently prognostic.
Inferential
[1] tests adding abiraterone to enzalutamide concurrently, so applying it to the sequential-switch claim is an inferential step.
The load-bearing papers from a corpus of 19, each classified by what it does to the claim. Every DOI is taken from the corpus the claim was graded on.
RCT showing abiraterone cannot overcome enzalutamide resistance — consistent with shared-pathway cross-resistance.
“Median progression-free survival was 5.7 months in the combination group and 5.6 months in the control group (hazard ratio, 0.83; 95% CI, 0.61 to 1.12; P = .22).”
Panel auditInternal validity: high · scope of inference: Chemotherapy-naive mCRPC with PSA progression on enzalutamide, testing whether combination overcomes resistance.Key limitations: combination design, not sequential monotherapy · no ARPI-naive comparator · enzalutamide-progressing enriched population
Prospective demonstration of clinical cross-resistance (enzalutamide after abiraterone), with reduced but non-zero activity (27% PSA response, rPFS 8.1 mo).
“Although cross-resistance between the two agents was observed in a majority of patients, some still benefited from enzalutamide treatment.”
Panel auditInternal validity: medium · scope of inference: mCRPC progressing after at least 24 weeks of abiraterone plus prednisone, receiving enzalutamide next.Key limitations: single-arm, no comparator · cross-resistance inferred clinically, mechanism not shown · unconfirmed PSA response
Shows sequence order affects combined PFS, consistent with partial cross-resistance between the two agents.
“Multivariable analysis suggested a difference between groups favoring the abiraterone-to-enzalutamide sequence with respect to combined PFS (HR 0.37, 95%CI 0.22-0.64, P < 0.001).”
Panel auditInternal validity: low · scope of inference: mCRPC treated with both agents sequentially, pre-chemotherapy, at a single institution.Key limitations: retrospective · imbalanced groups (65 vs 16) · no cross-resistance-free comparator
Control arm (switch to the other ARPI) shows poor rPFS of 3.6 mo, indirect support for limited second-ARPI efficacy.
“imaging-based progression-free survival was significantly longer in the olaparib group than in the control group (median, 7.4 months vs. 3.6 months; hazard ratio for progression or death, 0.34; 95% confidence interval, 0.25 to 0.47; P<0.001)”
Panel auditInternal validity: high · scope of inference: mCRPC with homologous recombination repair gene alterations progressing on one prior next-generation hormonal agent.Key limitations: control arm is ARPI switch · 81% crossover confounds the control arm · restricted to HRR-altered disease
4 of 19 papers shown. The full review lists all of them, with the panel’s arbitration →
Claims are graded at each quarterly release, and re-graded between releases only when a pivotal trial lands. Every change is a new version of the claim.
The corpus robustly carries the clinical cross-resistance phenomenon, but the strict mechanistic attribution to shared AR alterations is only partly established, holding the verdict at Likely rather than Established.
Release 2026-q4 re-graded every claim from its corpus under protocol v0.19. An earlier release's grade is shown for the record; it came from an earlier protocol and is not a movement.
Assumptions that carry weight but are not themselves established.
The panel records the gap that would most move the claim, and the study that would close it.
Causal role of AR alterations in patients. The claim's mechanistic clause rests on preclinical [14][17] and associative [15] data, while [9] questions AR markers' independent prognostic value. The panel rates this the highest-importance open gap for the claim, and names the study design that would close it: A prospective sequential-therapy cohort with serial AR genomic and AR-V7 profiling and prespecified multivariable modelling against tumour burden and TP53, testing whether acquired AR alterations predict second-agent failure.
Open question: Do acquired AR pathway alterations causally mediate reduced second-ARPI efficacy, independent of tumour burden and TP53 status? Why it is still open: Mechanistic evidence is preclinical [14][17] or associative [15], and [9] found no single AR perturbation prognostic after multivariable adjustment.
Priors is a sceptical first reader, not peer review, and not medical advice.
Release 2026-q4 · graded 26 Oct 2026 · protocol v0.19