Absorption, Metabolism and Excretion of GBT440, a Novel Hemoglobin S (HbS) Polymerization Inhibitor for the Treatment of Sickle Cell Disease (SCD), in Healthy Male Subjects

Background: SCD is caused by a point mutation in the β-globin gene producing hemoglobin S (HbS) that polymerizes upon deoxygenation with subsequent formation of sickled red blood cells (RBCs). GBT440 is a novel, orally bioavailable small molecule that inhibits HbS polymerization by increasing the affinity of O₂ to hemoglobin (Hb).

Methods: The pharmacokinetics, mass balance, and metabolite profile of [¹⁴C]-GBT440 were evaluated in 7 healthy male subjects in this open-label study. In order to evaluate the disposition kinetics of GBT440 at steady-state concentrations, a loading/maintenance dose schema was employed. Each subject received an oral loading dose of 2000 mg GBT440 on Day 1 followed by oral maintenance doses of 400 mg once daily on Day 2 to Day 4. Once the target steady-state was achieved, a single [¹⁴C]-GBT440 400 mg dose (approximately 100 μCi) was administered orally on Day 5. Blood, plasma, urine and feces were collected serially up to 26 days postdose.

Results: There were no serious adverse events or discontinuations due to adverse events for any of the healthy subjects participating in this study. GBT440 reached C_max in plasma and whole blood with median time to maximum concentration (T_max) values of 2.00 hours in plasma and whole blood and in 6.00 hours in RBCs. After reaching C_max, GBT440 concentrations appeared to decline in a monophasic manner, with the terminal elimination phase for GBT440 in plasma, whole blood, and RBCs appearing to decline in a parallel manner, with geometric mean T_1/2 values of 98.0 hours in plasma, 66.3 hours in whole blood, and 65.8 hours in RBCs. This study achieved 98.0% average recovery of total radioactivity in urine and feces over the course of the study. Most of the administered radioactivity (88.2%) was recovered by 144 hours postdose (Day 7). GBT440 was eliminated primarily in feces (62.6% of the total radioactive dose) with urinary excretion accounting for 35.4% of the total radioactive dose.

In whole blood, the majority of the total radioactivity (TRA) was unchanged GBT440 (97.5%) while three metabolites accounted for the remaining TRA (2.5%). In plasma, unchanged GBT440 was the prominent circulating radioactive component, accounting for 48.8% of the TRA. Eleven circulating metabolites with corresponding radioactive peaks were identified. There was one major Phase II metabolite (GBT440 O-dealkylation-sulfation), accounting for 16.8% of the TRA. Two potential active metabolites were identified but only accounted for 2.5% of the dose in whole blood.

GBT440 was eliminated predominately in feces. Unchanged GBT440 was the most abundant radioactive component, accounting for 33.3% of the administered dose. Four metabolites were identified, each accounting for 5.62%, 2.66%, 1.66% and less than 6% of the dose in the 0-216-hr human feces. Urine was a relatively minor excretion route for GBT440 in humans. An average of 34.3% of the dose was recovered in the urine samples. Unchanged GBT440 accounted for 0.08% of the administered dose and the rest were metabolites. GBT440 glucuronidation and reduction-glucuronidation products, which are Phase II metabolites, were the most abundant metabolites in urine, accounting for a combined 9.22% of dose. Because GBT440 does not undergo renal elimination, patients with renal disorders should not experience changes in pharmacokinetics of GBT440.

Conclusions: Although GBT440 has high specific binding to hemoglobin, it was completely excreted from the body with a half-life of approximately three days in healthy subjects. Since the half-life of GBT440 was much shorter than RBC lifespan (~ 120 days), this supports the hypothesis that the binding between GBT440 to hemoglobin is a reversible process. Following an oral administration, approximately one-third of the dose was excreted as the unchanged drug into the feces (unabsorbed and/or via biliary excretion). Two-thirds of the administered dose was metabolized and excreted into urine and feces. The major metabolic pathway was via Phase I and Phase II metabolism. Because GBT440 was not excreted directly into the urine, the pharmacokinetics are unlikely to be affected in patients with renal disorders.