Breast Cancer Drug Capivasertib Triggers Rapid Insulin Resistance Within Days of First Dose
A case report reveals capivasertib induces severe insulin resistance within 24 hours, reversing during drug-free intervals.
Summary
A 43-year-old woman with metastatic breast cancer and no history of diabetes developed marked insulin resistance within one day of starting capivasertib, an AKT inhibitor used alongside fulvestrant. Her post-meal blood glucose spiked to 256 mg/dL, while fasting insulin and HOMA-IR rose sharply, indicating hyperinsulinemic insulin resistance rather than a failure of insulin secretion. Crucially, these changes fully reversed during the drug's scheduled 3-day off periods without medication. Over subsequent treatment cycles, the metabolic disturbance appeared to attenuate. This case highlights that AKT pathway inhibition has rapid, pronounced effects on insulin sensitivity even in metabolically healthy patients, and that postprandial glucose monitoring from day one of therapy is essential. The findings have broader implications for understanding the AKT pathway's central role in adult metabolic regulation.
Detailed Summary
Capivasertib is a targeted cancer therapy that inhibits AKT, a kinase central to the PI3K/AKT/mTOR signaling axis. While this pathway is frequently mutated in hormone receptor-positive breast cancer, AKT also sits at the heart of insulin-mediated glucose uptake in peripheral tissues. Blocking it therefore creates an on-target metabolic liability that this case report documents in striking detail.
A 43-year-old woman with metastatic breast cancer, no personal or family history of diabetes, and a pre-treatment HbA1c of 5.6% began capivasertib on the standard 4-days-on/3-days-off schedule. Within 24 hours of her first dose, her 2-hour postprandial glucose reached 256 mg/dL. By day 2, fasting insulin was 22.8 µU/mL and HOMA-IR was 5.80 — both well above normal — while C-peptide remained intact, confirming that the pancreas was still secreting insulin normally. The problem was purely one of peripheral insulin resistance.
During the drug-free interval, all markers normalized spontaneously: fasting glucose fell to 87 mg/dL, insulin to 5.4 µU/mL, and HOMA-IR to 1.16 — without any glucose-lowering medication. This on/off pattern tracked the treatment schedule precisely, providing strong mechanistic evidence that AKT inhibition was directly responsible. Interestingly, HOMA-IR on day 4 declined across successive cycles (3.16 → 2.50 → 1.52), suggesting partial metabolic adaptation over time, though glycoalbumin continued to rise gradually to 17.9%.
For clinicians managing patients on AKT inhibitors, this report underscores that hyperglycemia can emerge within hours — not weeks — of initiation, even in non-diabetic patients. Postprandial glucose monitoring from the very first treatment day, timed to the dosing schedule, is warranted.
For the longevity-minded reader, this case is a vivid illustration of how essential AKT signaling is to normal insulin sensitivity in adults. The rapid, reversible insulin resistance mirrors the metabolic phenotype seen in aging and metabolic syndrome, reinforcing AKT as a key node in both cancer biology and age-related metabolic decline.
Key Findings
- Capivasertib caused postprandial glucose of 256 mg/dL and HOMA-IR of 5.80 within 24–48 hours of first dose.
- Insulin resistance fully reversed during 3-day drug-free intervals without glucose-lowering medication.
- C-peptide remained intact, confirming peripheral insulin resistance rather than impaired secretion.
- Metabolic effect appeared to attenuate across cycles: HOMA-IR on day 4 fell from 3.16 (cycle 1) to 1.52 (cycle 19).
- Glycoalbumin rose gradually to 17.9%, suggesting cumulative glycemic exposure despite cycle-linked reversibility.
Methodology
This is a single-patient case report from Juntendo University Urayasu Hospital, Japan. Glucose, insulin, C-peptide, HOMA-IR, and glycoalbumin were measured serially across multiple treatment cycles. No pre-treatment oral glucose tolerance test was performed, leaving baseline glucose tolerance status incompletely characterized.
Study Limitations
This summary is based on the abstract only, as the full text is not open access. As a single case report, findings cannot be generalized. The absence of a pre-treatment oral glucose tolerance test means pre-existing impaired glucose tolerance cannot be excluded, potentially confounding interpretation.
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