Potent inhibition of human organic cation transporter 2 (hOCT2) by β-carboline alkaloids

Abstract

1. Beta-carbolines are indole alkaloids with a wide range of pharmacological and toxicological activities. Beta-carbolines are structurally related to the neurotoxin 1-methyl-4-phenylpyridinium (MPP+), a known substrate of organic cation transporters (OCTs). The goal of this study is to determine the interaction of β-carbolines with human OCT1, 2, and 3 (SLC22A1-3).

2. Dose-dependent inhibition studies were performed for five commercially available β-carbolines using a fluorescent substrate assay in HEK293 cells stably expressing hOCT1-3. The substrate potential was evaluated by uptake assays and the impact of active transport on cellular toxicity examined.

3. All tested β-carbolines potently inhibited hOCT2 with IC₅₀ values in the sub- or low micromolar range. Harmaline is the most potent hOCT2 inhibitor (IC₅₀=0.50 ± 0.08 μM). hOCT1 and hOCT3 are less sensitive to β-carboline inhibition. Harmaline, norharmanium, and 2,9-dimethyl-4,9-dihydro-3H-β-carbolinium accumulated 2- to 7-fold higher in cells expressing hOCT1-3. HEK293 cells expressing hOCT1-3 were 6.5- to 13-fold more sensitive to harmane and norharmanium toxicity.

4. Our data support a significant role of hOCT1-3 in tissue uptake and disposition of β-carbolines. Importantly, the potent inhibition of hOCT2 by β-carbolines also raises the concern of potential drug interactions between naturally occurring bioactive alkaloids and drugs eliminated by hOCT2.

Keywords:

• β-Carbolines
• organic cation transporters
• hOCT2
• inhibitor
• drug interactions

Declaration of interest

This work was supported by the National Institutes of Health under Grants [DA032507], [GM066233], and [GM07750]. RWL was also partially supported by the Stipends for Training Aspiring Researchers (STAR) Program of the National Institutes of Health. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH. The authors declare no conflicts of interest.