Regorafenib (R) in advanced solitary fibrous tumor (SFT): Results from an exploratory phase II clinical study.
Authors: Stacchiotti Silvia , Baldi Giacomo Giulio , Lo Vullo Salvatore , Morosi Carlo , Greco Francesca Gabriella , Collini Paola , Barisella Marta , Dagrada Gianpaolo , Zaffaroni Nadia , Gronchi Alessandro , Simeone Noemi , Mariani Luigi , Frezza Anna Maria , Casali Paolo Giovanni 
Published: June 2021

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Background: R showed antitumor activity in a PDX model of dedifferentiated SFT (D-SFT), inducing a superior tumour growth inhibition than with other antiangiogenic agents (A), such as pazopanib (P) and axitinib (A). The efficacy of P and A in patients (pts) with advanced typical- (T-)/ malignant- (M-)SFT has been already confirmed in phase 2 clinical trials, but not in D-SFT. An exploratory phase 2 study was designed to investigate the activity of R in advanced SFT. Methods: An investigator-initiated exploratory phase 2 trial was started in December 2015 at the Istituto Nazionale Tumori, Milan, Italy, to evaluate the activity of R, 160 mg OD, 3 weeks on/1 week off, until progression or limiting toxicity. in > 18 years old pts with advanced SFT. The target sample size was 16 evaluable pts; at least 3 responses were requested to reject the null hypothesis of 5% in favour of an alternative hypothesis of 30% (with type-I and type-II error rates fixed at the 10% level). Eligible pts had to have evidence of progression. Prior treatment with A was allowed. Centralized pathologic review was performed, distinguishing T-SFT, M-SFT and D-SFT subtypes. The primary end-point was the overall response rate (ORR) by Choi. Secondary end-points were ORR by RECIST, progression-free survival (PFS), overall survival (OS). Results: Enrolment was completed in February 2021. Eighteen pts were enrolled (D-SFT = 4; M-SFT = 13; T-SFT = 1). Four pts were naïve, 14 were pre-treated [12 with antiangiogenics (4 with > 1 prior antiangiogenic line); 11 with cytotoxic agents]. Three pts are ongoing, 13 completed their treatment (11 = progression, 1 = toxicity, 1 = other). Fourteen pts are evaluable for response by Choi and RECIST (1 = screening failure; 1 = early discontinuation for toxicity before radiologic assessment; 2 = too early). A definitive dose reduction was required in 5 of 14 evaluable (35.7%) pts. The ORR by Choi was 42.9% (exact binomial 95% Confidence Interval [CI]: 17.7%-71.1%), with 6/14 (42.9%) partial responses (PR), 5/14 (35.7%) stable disease (SD) and 3/14 (21.4%) progressions (PD). Best responses by RECIST were: 1/14 (7.1%) PR, 10/14 (71.4%) SD, 3/14 (21.4%) PD. 5/6 pts responsive by Choi were pre-treated with another antiangiogenic. No responses were seen in the 3 D-SFT pts. At a m-FU of 23 months, m-PFS by Choi was 3.68 (IQR: 2.73-8.54) months, with 23.4% pts progression free at 1 year. m-PFS by Choi in responsive pts was 5.62 (IQR: 2.89 – 8.54) mos. Median OS was 15.7 (IQR: 7.35-not reached) months. Conclusions: R did not show a higher activity in D-SFT compared to P and A. The response rate was in the range observed with other A, but m-PFS was shorter. This may be due to discrepancies in pt populations and a high-rate of dose reductions with R. However, responses to R were seen also in pts previously treated with other A and almost one fourth of pts benefited from R for more than a year. Clinical trial information: 2015-002629-21.