Organic Anion Transporting Polypeptide 1B (OATP1B) Transporters Modulate Hydroxyurea Pharmacokinetics

Sickle cell is the most common inherited blood disorder affecting millions worldwide. Hydroxyurea is currently the only FDA approved drug for the treatment of sickle cell anemia (SCA) that alters disease pathophysiology. Known for inducing fetal hemoglobin production, hydroxyurea can decrease hospitalizations, vaso-occlusive pain episodes, and mortality associated with SCA. Although efficacious for many patients, a large degree of inter-patient variability in pharmacokinetic (PK) and pharmacodynamic (PD) properties of hydroxyurea has been noted, but sources of this variation remain largely undefined. Plasma transmembrane transporters have been shown to play an important role in the determination drug PK and PD of various xenobiotics. Our recent in vitro studies have demonstrated that hydroxyurea is a substrate for specific solute carrier (SLC) transporters including members of the organic anion transporting polypeptide 1B (OATP1B) subfamily which consists of OATP1B1, and OATP1B3 in humans, and one mouse orthologue Oatp1b2. These transporters are highly expressed on the sinusoidal membrane of hepatocytes, and function as uptake transporters bringing substrates from the blood into the liver. Limited reports have suggested that Oatp1b2 transcript is also found in kidney and intestine as well. In the present study we evaluated the uptake of the hydroxyurea by different haplotypes of hOATP1B3 transporter in vitro, as well as the influence of the Oatp1b2 transporter on hydroxyurea PK in vivo using a transporter knock-out mouse model.

Transporter-mediated cellular uptake of hydroxyurea was determined in vitro by measuring [¹⁴C]-hydroxyurea accumulation in Xenopus laevis oocytes that over express two common haplotypes of OATP1B3. hOATP1B3*1 and the functional variant hOATP1B3*2 that contains the 334T>G and 699G>A SNPs were assessed. In vivo PK Studies were performed using a knockout model mouse model in which mOATP1b2 was eliminated from the DBA/LcaJ1 background. Mice were dose with 50mg/kg of [¹⁴C]-hydroxyurea via oral gavage or intravenous (i.v.) injection. Serial blood samples were collected at defined timepoints and concluded with terminal blood and tissue collections at 2 hours post injection. Hydroxyurea levels were determined based on radioactivity in plasma and liver, and kidney homogenate. Accumulation in the urine was also measured over a 3 day period.

In vitro kinetic studies did not show any differences between OATP1B3*1 and OATP1B3*2 haplotypes in time-, or concentration-dependent accumulation of hydroxyurea. The inhibitor substrates naringen and rifampin decreased hydroxyurea accumulation by at least 50% in OATP1B3*1 but did not decrease accumulation in OATP1B3*2. These in vitro studies suggest that the genetic mutations associated with these two haplotypes do not have a functional impact on the transport of hydroxyurea, but may have significant implications for potential drug-drug interaction. In vivo, Hydroxyurea PK was altered in the absence of the Oatp1b2 transporter. Plasma levels of hydroxyurea were similar after i.v. injections, but total exposure (AUC) in Oatp1b2-KO mice was decreased by 20% with an AUC of 2257 mg-min/ml compared to 2872 mg-min/ml observed in the wildtype mice (p=0.04). Hydroxyurea levels in the kidneys of Oatp1b2-KO mice were approximately 50% of wildtype with a mean accumulation of 360 pmol/g tissue (p= 0.0025). In addition, 30% less hydroxyurea was excreted in urine of Oatp1b2-KO mice by 72 hours (p=0.023). Accumulation of hydroxyurea in liver of Oatp1b2-KO mice and wildtype mice were 107 and 132 pmol/g tissue respectively, but this difference was not statistically significant. These results suggest that hydroxyurea PK is altered in the absence of Oatp1b2 transporter after oral administration.

These studies provide the first in vivo evidence that specific transporters may be key determinants of hydroxyurea PK. Future studies are warranted to investigate the influence of human OATP1B and other transporters that modulate hydroxyurea PK. Identification of such transporters may help to elucidate factors such as genetic variations in the transporters and/or drug-drug interactions that may contribute to PK and PD variability observed in sickle cell patients.

No relevant conflicts of interest to declare.