The patients received oral rigosertib at a dose of 840 mg/day or higher on days 1-21 of a 28-day cycle. They also received azacitidine at 75 mg/m2 per day subcutaneously or intravenously for 7 days starting on day 8.
The median duration of treatment was 7.8 months in patients who were HMA naive and 4.9 months in patients who failed HMA therapy. The most common reasons for treatment discontinuation in the HMA-naive patients were toxicity (n = 8), progression (n = 7), and patient request (n = 7). The most common reasons for discontinuation in patients who had failed HMA therapy were progression (n = 12), toxicity (n = 5), and investigator decision (n = 4).
In total, 55 patients were evaluable for response, 26 who had failed HMA therapy and 29 who were HMA naive.
“The best responses, not surprisingly, were in patients that were HMA naive,” Dr. Woodman said.
In the HMA-naive patients, the overall response rate was 90%. Ten patients had a CR, five had a marrow CR with hematologic improvement, three had hematologic improvement alone, eight had a marrow CR alone, and three patients had stable disease. None of the patients progressed.
In the patients who had failed HMA therapy, the overall response rate was 54%. One patient achieved a CR, one had a partial response, five had a marrow CR with hematologic improvement, two had hematologic improvement alone, five had a marrow CR alone, seven had stable disease, and five progressed.
The median duration of response was 10.8 months in patients who failed HMA therapy and 12.2 months in the HMA-naive patients.
The most common TEAEs in the entire MDS cohort were hematuria (45%), constipation (43%), diarrhea (42%), fatigue (42%), dysuria (38%), pyrexia (36%), nausea (35%), neutropenia (31%), and thrombocytopenia (30%).
Grade 3 or higher TEAEs were neutropenia (27%), thrombocytopenia (26%), hematuria (9%), dysuria (9%), diarrhea (5%), fatigue (4%), and pyrexia (1%).
Dr. Woodman said patients who were most likely to be at risk for genitourinary toxicities (hematuria and dysuria) were those who weren’t well hydrated, took rigosertib at night, and didn’t void their bladders before bedtime. He said the researchers’ hypothesis is that there is some local bladder irritation in that setting.
However, the researchers found ways to mitigate the risk of genitourinary toxicities, including:
- Requiring the second dose of rigosertib to be taken in the afternoon rather than evening (about 3 p.m.).
- Asking patients to consume at least 2 liters of fluid per day.
- Having patients empty their bladders before bedtime.
- Assessing urine pH roughly 2 hours after the morning dose of rigosertib and prescribing sodium bicarbonate if the pH is less than 7.5.
Dr. Woodman said the phase 2 results in MDS patients have prompted the development of a phase 3 trial in which researchers will compare oral rigosertib plus azacitidine to azacitidine plus placebo.
Dr. Woodman is employed by Onconova Therapeutics, which sponsored the phase 1/2 trial. The Acute Leukemia Forum is held by Hemedicus, which is owned by the same company as this news organization.