CHICAGO – For patients with gastrointestinal stromal tumor (GIST) with KIT mutations conferring resistance to imatinib, a strategy of rapid alteration of drugs with complementary activity against KIT mutations is feasible but has thus far failed to yield significant clinical benefits, investigators said.
There were no objective responses among 12 patients treated continuously with 3 days of sunitinib (Sutent) followed by 4 days of regorafenib (Stivarga), and although 4 patients had stable disease in the short term, in each case the disease progressed within 16 weeks, reported Cesar Serrano-Garcia, MD, from the Dana-Farber Cancer Institute and Brigham and Women’s Hospital in Boston, and his colleagues.
“Drug exposure is critical to effectively target specific resistant subpopulations and low exposure may have contributed to the lack of efficacy in this cohort,” they wrote in a poster presented at the annual meeting of the American Society of Clinical Oncology.
The investigators noted that the main mechanism of resistance to imatinib (Gleevec) in GIST is polyclonal emergence of KIT secondary mutations. They then theorized that rapid alteration of sunitinib with regorafenib, which both have complementary activity against different KIT resistance mutations, could be a novel therapeutic strategy for controlling imatinib-resistant disease.
Both agents are active against KIT and platelet-derived growth factor receptor alpha (PDGFR-alpha). Sunitinib has stronger activity against ATP-binding pocket mutations, and regorafenib is more effective against activation loop oncoproteins, the investigators explained.
They conducted a phase Ib trial to evaluate the safety and preliminary efficacy of the strategy in patients with metastatic GIST that had advanced on therapy with all established protocols. The trial had a standard 3+3 design to determine the recommended phase 2 dose; a total of 14 patients were enrolled, but only 12 received one or more complete cycles.
The median patient age was 63.5%. Nine patients had Eastern Cooperative Oncology Group performance status of 0, and five had an ECOG status of 1. The patients had received a median of four prior lines of therapy, and all had received at least three lines.
The primary mutations were at KIT exon 11 in eight patients, exon 9 in five patients, and a KIT/PDGFR-alpha wild type in one patient.
Of the 12 patients who received one or more complete cycles, 7 were treated with sunitinib 37.5 mg daily for 3 days, followed by regorafenib 120 mg daily for 4 days. There were no dose-limiting toxicities in this group. The median number of cycles delivered was 2 (range 1-4).
The other five patients were treated with sunitinib at the same 37.5 mg daily dose for 3 days, followed immediately by regorafenib 160 mg daily for 4 days. There were two dose-limiting toxicities in this group, both grade 3 hypophosphatemia, one of which was refractory to phosphorous replacement.
Antitumor activity according to Response Evaluation Criteria in Solid Tumors version 1.1 included four cases of stable disease at the time of the efficacy analysis, and eight cases of disease progression. The median progression-free survival was 1.9 months. As noted before, there were no complete or partial responses among the 12 patients.
A pharmacokinetic profile at cycle 1 showed that neither drug reached its reported active blood drug levels.
The patients appeared to tolerate the treatment well, with grade 1 or 2 fatigue in all patients being the most common adverse events. Grade 3 or 4 events included hand-foot syndrome, hypertension, and hypophosphatemia in two patients each.
As noted, the authors acknowledged that low drug exposure levels may explain the lack of any responses in this cohort.
“Therapeutic strategies based on KIT inhibition remain crucial in GIST patients progressing to multiple lines,” they wrote.
The study was supported by an ASCO Young Investigator Award, Pfizer, and Bayer. Dr. Serrano-Garcia disclosed honoraria from Bayer, a consulting or advisory role for Deciphera, research funding from Bayer and Deciphera, and travel accommodations and expenses from Pfizer.
SOURCE: Serrano-Garcia C et al. ASCO 2018, Abstract 11510.