Mass balance and pharmacokinetics of an oral dose of 14 C‐napabucasin in healthy adult male subjects

This phase 1, open‐label study assessed 14 C‐napabucasin absorption, metabolism, and excretion, napabucasin pharmacokinetics, and napabucasin metabolites (primary objectives); safety/tolerability were also evaluated. Eight healthy males (18–45 years) received a single oral 240‐mg napabucasin dose containing ~100 μCi 14 C‐napabucasin. Napabucasin was absorbed and metabolized to dihydro‐napabucasin (M1; an active metabolite [12.57‐fold less activity than napabucasin]), the sole major circulating metabolite (median time to peak concentration: 2.75 and 2.25 h, respectively). M1 plasma concentration versus time profiles generally mirrored napabucasin; similar arithmetic mean half‐lives (7.14 and 7.92 h, respectively) suggest M1 formation was rate limiting. Napabucasin systemic exposure (per C max and AUC) was higher than M1. The total radioactivity (TRA) whole blood:plasma ratio (AUC last : 0.376; C max : 0.525) indicated circulating drug‐related compounds were essentially confined to plasma. Mean TRA recovery was 81.1% (feces, 57.2%; urine, 23.8%; expired air, negligible). Unlabeled napabucasin and M1 recovered in urine accounted for 13.9% and 11.0% of the dose (sum similar to urine TRA recovered); apparent renal clearance was 8.24 and 7.98 L/h. No uniquely human or disproportionate metabolite was quantified. Secondary glucuronide and sulfate conjugates were common urinary metabolites, suggesting napabucasin was mainly cleared by reductive metabolism. All subjects experienced mild treatment‐emergent adverse events (TEAEs), the majority related to napabucasin. The most commonly reported TEAEs were gastrointestinal disorders. There were no clinically significant laboratory, vital sign, electrocardiogram, or physical examination changes. Napabucasin was absorbed, metabolized to M1 as the sole major circulating metabolite, and primarily excreted via feces. A single oral 240‐mg dose was generally well tolerated.