BOSTON – An experimental agent that targets the byproducts of gene fusions has shown surprising clinical activity against notoriously treatment-refractory cancers in early results from a phase I trial.
“I think that’s what remarkable about this is that we have all these different histologies and many different fusions, but all seem to be having some kind of [response] to this molecule,” said lead investigator Dr. David S. Hong, deputy chair and associate professor in the department of investigational cancer therapeutics at the University of Texas MD Anderson Cancer Center, Houston.
The molecule, LOXO-101 is a selective inhibitor of abnormal TRKA, TRKAB, and TRKC kinases that arise from gene fusions. TRK fusions have been implicated in tumor development in preclinical studies, he reported at the AACR–NCI–EORTC International Conference on Molecular Targets and Cancer Therapeutics.
One patient, a 41-year-old woman with undifferentiated soft tissue sarcoma of the groin that had metastasized to her lungs had rapid and “substantial” tumor and lung-nodule regression that radiographically fell just shy of a complete response, Dr. Hong said.
The drug also induced robust partial responses in a 55-year-old man with a gastrointestinal stromal tumor (GIST) for whom five prior targeted agents had failed, and a 33-year-old man with mammary analogue secretory carcinoma of the salivary gland (MASC) that had progressed on chemotherapy with docetaxel, carboplatin, and 5-fluorauracil.
A fourth patient, a man (age unspecified) who had papillary thyroid cancer with palpable lymphadenopathy, had no palpable nodes at 1-month follow-up, and appears to be responding to the drug, Dr. Hong added.
“This is really a dramatic example, I think, of a targeted therapy that is not histology specific, and this is another area in oncology where it’s new, and we’re just feeling our way,” commented Dr. Lee C. Helman, a sarcoma specialist at the National Cancer Institute in Bethesda, Md., who moderated the briefing where Dr. Hong presented the data.
The TRK family of proteins consists of separate tyrosine kinases that, when functioning normally, are important for the development of peripheral central nervous system processes such as pain and thermoregulation, Dr. Hong explained.
“The NTRK 1, 2, and 3 genes which encode for the TRK proteins are subject to these gene rearrangements and fusions, and the resulting TRK gene fusions may become erroneously expressed, and the kinase domain constitutively activated,” he said.
Gene fusions resulting in constitutively active TRKA, B, and C kinases can occur in the range of about 2%-25% in a wide variety of tumors, including adenocarcinomas of the lung, gliomas, thyroid tumors, head and neck cancers, sarcomas, and other types. These fusions have also been detected in more than 75% of MASC tumors, secretory breast carcinomas, and infantile (congenital) fibrosarcoma.
LOXO-101 is an orally delivered small molecule that was rationally designed to target the TRK fusions while largely leaving other kinases alone. In preclinical studies, the compound showed potent tumor growth inhibition and regression in mice bearing NTRK fusions.
The phase I trial is a dose-finding study including a total of 24 patients with various treatment-refractory advanced or metastatic solid tumors. Patients were enrolled regardless of fusional status.
At the selected dose of 100 mg twice daily, patients appeared to tolerate the drug very well, with few off-target adverse events, Dr. Hong said. The most common side effect was mild dizziness. Grade 3 or 4 adverse events occurring at all doses levels include fatigue (one patient), anemia (two), abdominal pain (one), increased alkaline phosphatase (one), increased aspartate aminotransferase, delirium, and syncope (two each).
A total of six of the 24 patients had TRK fusions in their tumors, and three of these patients were available for assessment as of Oct. 20, 2015.
Dr. Hong and colleagues previously reportedon the case of the 41-year old woman with sarcoma. She was found to have a fusion of LMNA (a gene that encodes for nuclear membrane proteins) with NTRK1. She was started on the 100-mg b.i.d. dose of LOX-101 and had rapid resolution of dyspnea and hypoxemia. She had a confirmed partial response, and a CT scan showed that her multiple pulmonary nodules had regressed, with just a single, small disease site remaining at most recent follow-up. This patient continues on treatment and has been followed for more than 8 months.
The man with GIST had experienced disease progression while on therapy with imatinib, sunitinib, sorafenib, nilotinib, and regorafenib. He was treated in the 150-mg b.i.d. dose cohort, had a confirmed partial response, and remains on study after 4-plus months.