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
Authors: Rademacher Peter , Hutchaleelaha Athiwat , Washington Carla , Lehrer Josh , Ramos Eleanor 
Published: October 2019

Abstract
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 Cmax in plasma and whole blood with median time to maximum concentration (Tmax) values of 2.00 hours in plasma and whole blood and in 6.00 hours in RBCs. After reaching Cmax, 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 T1/2 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.

Disclosures
Rademacher: Global Blood Therapeutics: Employment, Equity Ownership. Hutchaleelaha:Global Blood Therapeutics: Employment, Equity Ownership. Washington:Global Blood Therapeutics: Employment, Equity Ownership. Lehrer:Global Blood Therapeutics: Employment, Equity Ownership. Ramos:Global Blood Therapeutics: Employment, Equity Ownership.