L-carnitine as a Probe to Assess Mitochondrial Drug Toxicity

Abstract

Adverse drug reactions (ADRs) are an inherent risk of any drug therapy regimen. Despite significant advances in medical sciences and constant improvements in the drug development process, ADRs still represent a major healthcare concern for patients and clinicians as well as a burden on the healthcare system. Recently, mitochondrial metabolites have become of interest because of their potential use as signaling molecules for mitochondrial health. From both a patient and healthcare perspective, there are benefits in identifying mitochondria-specific metabolites that could be clinically employed to probe and assess mitochondrial related alterations in metabolism. The mechanisms mediating the pharmacokinetics and disposition of mitochondrial substrates and metabolites can be used to identify increased toxicity risk specifically due to mitochondrial dysfunction caused by ADRs. With this rationale, this dissertation looked to (1) apply a database mining strategy to identify candidate mitochondrial metabolites that could be clinically employed to identify individuals at increased risk of mitochondrial-related ADRs, (2) to demonstrate how an L-carnitine challenge can be useful as a metabolic stress biomarker to identify individuals at increased risk of mitochondrial-related ADRs, and (3) to establish differences between male and female CFZ treated mice and associated interactions with L-carnitine. To achieve this, a database screening strategy with an a priori established criteria was employed to identify candidate mitochondrial metabolites that could be clinically useful to identify increased risk of mitochondrial-related ADRs. Four publicly available databases were used and allowed for a systematic process of elimination, which led to a concise list of clinically feasible human metabolite candidates for use as functional tracers of mitochondrial metabolic health. L-carnitine was identified as a candidate mitochondrial metabolite. We then hypothesized that an intravenous dose of exogenous L-carnitine- an “L-carnitine challenge test”- could identify individuals at risk for mitochondrial-related ADRs by provoking variation in subsequent measurement of serial L-carnitine and/or acetylcarnitine blood concentrations. To test this, an established animal model of mitochondrial drug toxicity was used in which male C57BL/6 mice were treated with clofazimine (CFZ) by its addition to their chow for 8-weeks. Following CFZ treatment, mice were injected with a high dose (1,000 mg/kg) of L-carnitine. Metabolic functions were tracked, and urine and blood samples were assayed for L-carnitine and acetylcarnitine concentrations using a quantitative LC/MS analysis. The L-carnitine challenge test identified CFZ-dependent differences in whole blood acetylcarnitine concentration. This finding supports and substantiates the potential of the challenge test as a “probe” to identify drug-related toxicological manifestations. Lastly, we looked to further establish L-carnitine feasibility for clinical use in both male and female, and to aid in further closing the knowledge gap of sex-related differences in ADR incidence, since females are found to experience ADRs at a higher rate than males. The “L-carnitine challenge test” was once again used, this time in both male and female mice. The results from this project showed that the L-carnitine “challenge test” induced differences in whole blood acetylcarnitine concentrations in both CFZ-treated male and female mice. This dissertation serves as a starting point in the development of an “L-carnitine challenge” test a clinically useful probe to identify drug-related toxicological manifestations, with the goal of potentially reducing ADRs incidences during lifesaving drug treatment.