Tumor cell infiltration is a major mechanism of treatment escape in glioblastoma. Src is an intracellular tyrosine kinase that mediates tumor cell motility and invasiveness. We evaluated the efficacy and safety of bosutinib, a tyrosine kinase inhibitor that potently inhibits Src and Abl, in patients with recurrent glioblastoma. In this two-arm study, patients with histologically confirmed recurrent glioblastoma and ≤2 relapses, not previously treated with anti-vascular endothelial growth factor (VEGF) therapy, were administered oral bosutinib 400 mg daily. Arm A planned for 6 patients who were candidates for surgical resection to be given bosutinib for 7-9 days prior to resection. Arm B was a two-stage design phase 2 trial targeting 30 patients. The primary endpoint was progression-free survival at 6 months (PFS6) in Arm B. After 9 patients enrolled onto stage 1 of Arm B, 9 (100 %) patients progressed within 6 months. Therefore, the study met the pre-specified criteria for early closure and both Arms were closed. In Arm B, Median PFS was 7.71 weeks and median OS was 50 weeks. Best objective response was stable disease in one patient (11.1 %). Seven patients (77.8 %) had treatment-related AEs of any grade and 2 (22.2 %) were grade ≥3. Arm A was closed after 2 patients enrolled. Src activation was evident in all archival tumor samples. Bosutinib monotherapy does not appear to be effective in recurrent glioblastoma. However, Src remains a potential target based on its upregulation in tumor samples and role in glioma invasion.

Background

Inhibition of epidermal growth factor receptor (EGFR) and the mechanistic target of rapamycin (mTOR) may have synergistic antitumor effects in high-grade glioma patients.

Methods

We conducted a phase I/II study of the EGFR inhibitor erlotinib (150 mg/day) and the mTOR inhibitor temsirolimus. Patients initially received temsirolimus 50 mg weekly, and the dose adjusted based on toxicities. In the phase II component, the primary endpoint was 6-month progression-free survival (PFS6) among glioblastoma patients.

Results

Twenty-two patients enrolled in phase I, 47 in phase II. Twelve phase I patients treated at the maximum tolerated dosage were included in the phase II cohort for analysis. The maximum tolerated dosage was 15 mg temsirolimus weekly with erlotinib 150 mg daily. Dose-limiting toxicities were rash and mucositis. Among 42 evaluable glioblastoma patients, 12 (29%) achieved stable disease, but there were no responses, and PFS6 was 13%. Among 16 anaplastic glioma patients, 1 (6%) achieved complete response, 1 (6%) partial response, and 2 (12.5%) stable disease, with PFS6 of 8%. Tumor levels of both drugs were low, and posttreatment tissue in 3 patients showed no reduction in the mTOR target phosphorylated (phospho-)S6(S235/236) but possible compensatory increase in phospho-Akt(S473). Presence of EGFR variant III, phospho-EGFR, and EGFR amplification did not correlate with survival, but patients with elevated phospho-extracellular signal-regulated kinase or reduced phosphatase and tensin homolog protein expression had decreased progression-free survival at 4 months.

Conclusion

Because of increased toxicity, the maximum tolerated dosage of temsirolimus in combination with erlotinib proved lower than expected. Insufficient tumor drug levels and redundant signaling pathways may partly explain the minimal antitumor activity noted.