Biological Distribution after Oral Administration of Radioiodine-Labeled Acetaminophen to Estimate Gastrointestinal Absorption Function via OATPs, OATs, and/or MRPs

Organic anion transporting polypeptides: Pharmacology, toxicology, structure, and transport mechanisms

Organic anion transporting polypeptides (OATPs) are membrane proteins that mediate the uptake of a wide range of substrates across the plasma membrane of various cells and tissues. They are classified into 6 subfamilies, OATP1 through OATP6. Humans contain 12 OATPs encoded by 11 solute carrier of organic anion transporting polypeptide (SLCO) genes: OATP1A2, OATP1B1, OATP1B3, the splice variant OATP1B3-1B7, OATP1C1, OATP2A1, OATP2B1, OATP3A1, OATP4A1, OATP4C1, OATP5A1, and OATP6A1. Most of these proteins are expressed in epithelial cells, where they mediate the uptake of structurally unrelated organic anions, cations, and even neutral compounds into the cytoplasm. The best-characterized members are OATP1B1 and OATP1B3, which have an important role in drug metabolism by mediating drug uptake into the liver and are involved in drug-drug interactions. In this review, we aimed to (1) provide a historical perspective on the identification of OATPs and their nomenclature and discuss their phylogenic relationships and molecular characteristics; (2) review the current knowledge of the broad substrate specificity and their role in drug disposition and drug-drug interactions, with a special emphasis on human hepatic OATPs; (3) summarize the different experimental systems that are used to study the function of OATPs and discuss their advantages and disadvantages; (4) review the available experimental 3-dimensional structures and examine how they can help elucidate the transport mechanisms of OATPs; and (5) finally, summarize the current knowledge of the regulation of OATP expression, discuss clinically important single-nucleotide polymorphisms, and outline challenges of physiologically based pharmacokinetic modeling and in vitro to in vivo extrapolation. SIGNIFICANCE STATEMENT: Organic anion transporting polypeptides (OATPs) are a family of 12 uptake transporters in the solute carrier superfamily. Several members, particularly the liver-expressed OATP1B1 and OATP1B3, are important drug transporters. They mediate the uptake of several endobiotics and xenobiotics, including statins and numerous other drugs, into hepatocytes, and their inhibition by other drugs or reduced expression due to single-nucleotide polymorphisms can lead to adverse drug effects. Their recently solved 3-dimensional structures should help to elucidate their transport mechanisms and broad substrate specificities.

Massive acetaminophen ingestion managed successfully with N-acetylcysteine, fomepizole, and renal replacement therapy

Acetaminophen ingestion is routinely managed with the antidote, N-acetylcysteine (NAC). Massive acetaminophen poisoning has been treated successfully with adjunctive therapies such as fomepizole and hemodialysis. Fomepizole functions by inhibiting cytochrome p560, which prevents tylenol from forming its toxic metabolite, NAPQI. Prior cases have demonstrated favorable outcomes and a significant drop in acetaminophen levels after a single session of intermittent hemodialysis and continuous veno-venous hemofiltration (CVVH). However, the recommended dosage adjustments of NAC and fomepizole while a patient is undergoing CVVH has not been well reported. We present a case of an 18-year-old male who presented after ingesting 125 g of tylenol. His 4-hour acetaminophen level was 738.6 µg/mL. He was treated with NAC, fomepizole, and a single 4-hour session of hemodialysis. His acetaminophen level remained elevated at 730 µg/mL despite the hemodialysis session. CVVH was initiated, and he was given intravenous NAC at 12.5 mg/kg/h, oral NAC at 70 mg/kg every 4 hours, and intravenous fomepizole at 10 mg/kg every 6 hours. His tylenol levels became undetectable 57 hours after ingestion, and he did not develop permanent liver toxicity. This case encourages the use of CVVH for massive tylenol ingestion when a single run of intermittent hemodialysis is not effective in lowering the tylenol level. NAC, fomepizole, and CVVH can prevent unfavorable outcomes in massive acetaminophen ingestion when provided at an appropriate dose and frequency.