Results from a phase I study of 4-l-[131I]iodo-phenylalanine ([131I]IPA) with external radiation therapy in patients with recurrent glioblastoma (IPAX-1).

Background: Glioblastoma (GBM), the most common malignant brain tumor, is associated with devastating outcomes. IPAX-1 was a multicenter, open-label, single-arm phase I study to evaluate carrier-added 4-L-[¹³¹I]iodo-phenylalanine ([¹³¹I]IPA) plus external radiation therapy (XRT) in recurrent GBM.

Methods: A total of 10 adults with recurrent GBM who had received first-line debulking surgery plus radio-chemotherapy, were randomized to a single-dose regimen (1f; ¹³¹I-IPA 2 GBq before XRT); a fractionated parallel dose regimen (3f-p; 3 ¹³¹I-IPA 670 MBq fractions, in parallel with second-line XRT), or a fractionated sequential dose regimen (3f-s; 3 ¹³¹I-IPA 670 MBq fractions before and after XRT). Metabolic tumor responses were determined using O-(2-[¹⁸F]fluoroethyl)-l-tyrosine positron emission tomography, while single-photon emission computed tomography was used to guide [¹³¹I]IPA tumor dosimetry.

Results: All dose regimens were well tolerated. Organ-absorbed radiation doses in red marrow (0.38 Gy) and kidney (1.28 Gy) confirmed no radiation-based toxicity. Stable disease was observed in 4 of the 9 patients at 3 months post-treatment (3-month follow-up [FU], 1 patient did not reach protocol-mandated end of study), yielding a response rate of 44.4%. At the 3-month FU, 6 patients demonstrated metabolic stable disease. Median progression-free survival was 4.3 months (95% confidence interval [CI]: 3.3-4.5), while median overall survival was 13 months (95% CI: 7.1-27).

Conclusions: Single or fractionated doses of [¹³¹I]IPA plus XRT were associated with acceptable tolerability and specific tumor targeting in patients with recurrent GBM, warranting further investigation.