Cytarabine (Ara‐C) is a nucleoside analog used in the treatment of acute myeloid leukemia (AML). Despite the many years of clinical use, the identity of the transporter(s) involved in the disposition of Ara‐C remains poorly studied. Previous work demonstrated that concurrent administration of Ara‐C with nitrobenzylmercaptopurine ribonucleoside (NBMPR) causes an increase in Ara‐C plasma levels, suggesting involvement of one or more nucleoside transporters. Here, we confirmed the presence of an NMBPR‐mediated interaction with Ara‐C resulting in a 2.5‐fold increased exposure. The interaction was unrelated to altered blood cell distribution, and subsequent studies indicated that the disposition of Ara‐C was unaffected in mice with a deficiency of postulated candidate transporters, including ENT1, OCTN1, OATP1B2, and MATE1. These studies indicate the involvement of an unknown NBMPR‐sensitive Ara‐C transporter that impacts the pharmacokinetic properties of this clinically important agent.

Murine pharmacokinetics (PK) represents the absorption, distribution, metabolism, and elimination of drugs from the body, which helps to guide clinical studies, ultimately resulting in more effective drug treatment. The purpose of this protocol is to describe a serial bleeding protocol, obtaining blood samples at six time points from single mouse to yield a complete PK profile. This protocol has proved to be rapid, highly repeatable, and relatively easy to acquire. Comparing with the conventional PK studies, this method not only dramatically reduces animal usage, but also decreases sample variation obtained from different animals.

Reduced expression of the uptake transporter, OCTN1 (SLC22A4), has been reported as a strong predictor of poor event-free and overall survival in multiple cohorts of patients with acute myeloid leukemia (AML) receiving the cytidine nucleoside analog, cytarabine (Ara-C). To further understand the mechanistic basis of interindividual variability in the functional expression of OCTN1 in AML, we hypothesized a mechanistic connection to DNA methylation-based epigenetic repression of SLC22A4. We found increased basal SLC22A4 methylation was associated with decreased Ara-C uptake in AML cell lines. Pre-treatment with hypomethylating agents, 5-azacytidine, or decitabine, restored SLC22A4 mRNA expression, increased cellular uptake of Ara-C, and was associated with increased cellular sensitivity to Ara-C compared with vehicle-treated cells. Additionally, lower SLC22A4 methylation status was associated with distinct clinical advantages in both adult and pediatric patients with AML. These findings suggest a regulatory mechanism is involved in the interindividual variability in response to Ara-C, and provides a basis for the integration of hypomethylating agents into Ara-C-based treatment regimens. Acute myeloid leukemia (AML) is a blood disorder that is classified by abnormal proliferation and differentiation of myeloid cells within the bone marrow compartment. Despite advances in supportive care, the backbone of therapy has remained unchanged for the last 30 years, and consists of combination regimens of cytarabine (Ara-C) and anthracyclines. 1 The efficacy and response to Ara-C-based treatment vary dramatically between individual patients with AML and are, in part, dependent on efficiency of Ara-C uptake, 2 intracellular activation, 3 and deamination. 4 Because the transport of Ara-C is the initial step to intracellular accumulation and subsequent cytotoxicity, defective uptake is a major contributor to clinical resistance observed with nucleoside analog-based therapy in AML. 5 One hallmark of AML is dysregulation at the genetic and epigenetic level. 6,7 Previously completed clinical trials have

Regorafenib, a multikinase inhibitor used in the treatment of various solid tumors, undergoes extensive uridine 5′‐diphosphate glucuronosyltransferase (Ugt)1a9‐mediated glucuronidation to form regorafenib‐N‐β‐glucuronide (M7; RG), but the contribution of hepatic uptake transporters, such as organic anion‐transporting polypeptide (Oatp)1b2, to the pharmacokinetics of regorafenib remains poorly understood. Using NONMEM‐based, population‐based, parent‐metabolite modeling, we found that Oatp1b2 and sex strongly impact the systemic exposure to RG in mice receiving oral regorafenib. Metabolic studies revealed that the liver microsomal expression of cytochrome P450 (Cyp)3a11 is twofold lower in female mice, whereas Ugt1a9 levels and function are not sex dependent. This finding is consistent with the metabolism of regorafenib occurring via two competing pathways, and the lack of Oatp1b2 results in decreased clearance of RG. The described model provides mechanistic insights into the in vivo disposition of regorafenib.

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