In vitro characterization of the human biotransformation pathways of aplidine, a novel marine anti-cancer drug

CYP3A-Mediated Carbon-Carbon Bond Cleavages in Drug Metabolism

Cytochrome P450 enzymes (P450s) play a critical role in drug metabolism, with the CYP3A subfamily being responsible for the biotransformation of over 50% of marked drugs. While CYP3A enzymes are known for their extensive catalytic versatility, one intriguing and less understood function is the ability to mediate carbon-carbon (C-C) bond cleavage. These uncommon reactions can lead to unusual metabolites and potentially influence drug safety and efficacy. This review focuses on examining examples of C-C bond cleavage catalyzed by CYP3A, exploring the mechanisms, physiological significance, and implications for drug metabolism. Additionally, examples of CYP3A-mediated ring expansion via C-C bond cleavages are included in this review. This work will enhance our understanding of CYP3A-catalyzed C-C bond cleavages and their mechanisms by carefully examining and analyzing these case studies. It may also guide future research in drug metabolism and drug design, improving drug safety and efficacy in clinical practice.

Unusual Biotransformation Reactions of Drugs and Drug Candidates

Detailed assessment of the fate of drugs in nonclinical test species and humans is essential to ensure the safety and efficacy of medicines in patients. In this context, biotransformation of drugs and drug candidates has been an area of keen interest over many decades in the pharmaceutical industry as well as academia. Although many of the enzymes and biotransformation pathways involved in the metabolism of xenobiotics and more specifically drugs have been well characterized, each drug molecule is unique and constitutes specific challenges for the biotransformation scientist. In this mini-review written for the special issue on the occasion of the 50th Anniversary celebration of Drug Metabolism and Disposition and to celebrate contributions of F. Peter Guengerich, one of the pioneers of the drug metabolism field, recently reported "unusual" biotransformation reactions are presented. Scientific and technological advances in the "toolbox" of the biotransformation scientists are summarized. As the pharmaceutical industry continues to explore therapeutic modalities different from the traditional small molecule drugs, the new challenges confronting the biotransformation scientist as well as future opportunities are discussed. SIGNIFICANCE STATEMENT: For the biotransformation scientists, it is essential to share and be aware of unexpected biotransformation reactions so that they can increase their confidence in predicting metabolites of drugs in humans to ensure the safety and efficacy of these metabolites before the medicines reach large numbers of patients. The purpose of this review is to highlight recent observations of "unusual" metabolites so that the scientists working in the area of drug metabolism can strengthen their readiness in expecting the unexpected.

The FDA-Approved Drug Cobicistat Synergizes with Remdesivir To Inhibit SARS-CoV-2 Replication In Vitro and Decreases Viral Titers and Disease Progression in Syrian Hamsters

Combinations of direct-acting antivirals are needed to minimize drug resistance mutations and stably suppress replication of RNA viruses. Currently, there are limited therapeutic options against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and testing of a number of drug regimens has led to conflicting results. Here, we show that cobicistat, which is an FDA-approved drug booster that blocks the activity of the drug-metabolizing proteins cytochrome P450-3As (CYP3As) and P-glycoprotein (P-gp), inhibits SARS-CoV-2 replication. Two independent cell-to-cell membrane fusion assays showed that the antiviral effect of cobicistat is exerted through inhibition of spike protein-mediated membrane fusion. In line with this, incubation with low-micromolar concentrations of cobicistat decreased viral replication in three different cell lines including cells of lung and gut origin. When cobicistat was used in combination with remdesivir, a synergistic effect on the inhibition of viral replication was observed in cell lines and in a primary human colon organoid. This was consistent with the effects of cobicistat on two of its known targets, CYP3A4 and P-gp, the silencing of which boosted the in vitro antiviral activity of remdesivir in a cobicistat-like manner. When administered in vivo to Syrian hamsters at a high dose, cobicistat decreased viral load and mitigated clinical progression. These data highlight cobicistat as a therapeutic candidate for treating SARS-CoV-2 infection and as a potential building block of combination therapies for COVID-19.

Metabolite profiling of the novel anti-cancer agent, plitidepsin, in urine and faeces in cancer patients after administration of 14C-plitidepsin

PURPOSE

Plitidepsin absorption, distribution, metabolism and excretion characteristics were investigated in a mass balance study, in which six patients received a 3-h intravenous infusion containing 7 mg ¹⁴C-plitidepsin with a maximum radioactivity of 100 µCi.

METHODS

Blood samples were drawn and excreta were collected until less than 1% of the administered radioactivity was excreted per matrix for two consecutive days. Samples were pooled within-patients and between-patients and samples were screened for metabolites. Afterwards, metabolites were identified and quantified. Analysis was done using Liquid Chromatography linked to an Ion Trap Mass Spectrometer and offline Liquid Scintillation Counting (LC-Ion Trap MS-LSC).

RESULTS

On average 4.5 and 62.4% of the administered dose was excreted via urine over the first 24 h and in faeces over 240 h, respectively. Most metabolites were found in faeces.

CONCLUSION

Plitidepsin is extensively metabolised and it undergoes dealkylation (demethylation), oxidation, carbonyl reduction, and (internal) hydrolysis. The chemical formula of several metabolites was confirmed using high resolution mass data.

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.

Pharmacokinetics and excretion of 14C-Plitidepsin in patients with advanced cancer

Plitidepsin (Aplidin®) is a marine-derived anticancer compound currently investigated in phase III clinical trials. This article describes the distribution, metabolism and excretion of this novel agent and it mainly aims to identify the major routes of elimination. Six subjects were enrolled in a mass balance study during which radiolabelled plitidepsin was administered as a 3-h intravenous infusion. Blood samples were taken and urine and faeces were collected. Total radioactivity (TRA) analysis using Liquid Scintillation Counting (LSC) was done to determine the amount of radioactivity excreted from the body and plitidepsin concentrations in whole blood, plasma and urine were determined by validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays. In total, a mean of 77.4% of the administered radioactivity was excreted over a time period of 20 days, of which 71.3% was recovered in faeces and 6.1% was found in urine. The majority excreted in urine was accounted for by unchanged plitidepsin, with only 1.5% of the total administered dose explained by metabolites in urine. Faeces, on the other hand contained low levels of parent compound, which means that most of the TRA excreted in faeces was accounted for by metabolites. TRA levels were 3.7 times higher in whole blood compared to plasma. Plitidepsin was widely distributed and plasma clearance was low. This study shows that red blood cells are a major distribution compartment and that the biliary route is the main route of total radioactivity excretion.

Carbon-carbon bond cleavage and formation reactions in drug metabolism and the role of metabolic enzymes

Elimination of xenobiotics from the human body is often facilitated by a transformation to highly water soluble and more ionizable molecules. In general, oxidation-reduction, hydrolysis, and conjugation reactions are common biotransformation reactions that are catalyzed by various metabolic enzymes including cytochrome P450s (CYPs), non-CYPs, and conjugative enzymes. Although carbon-carbon (C-C) bond formation and cleavage reactions are known to exist in plant secondary metabolism, these reactions are relatively rare in mammalian metabolism and are considered exceptions. However, various reactions such as demethylation, dealkylation, dearylation, reduction of alkyl chain, ring expansion, ring contraction, oxidative elimination of a nitrile through C-C bond cleavage, and dimerization, and glucuronidation through C-C bond formation have been reported for drug molecules. Carbon-carbon bond cleavage reactions for drug molecules are primarily catalyzed by CYP enzymes, dimerization is mediated by peroxidases, and C-glucuronidation is catalyzed by UGT1A9. This review provides an overview of C-C bond cleavage and formation reactions in drug metabolism and the metabolic enzymes associated with these reactions.

Mussels increase xenobiotic (azaspiracid) toxicity using a unique bioconversion mechanism

Azaspiracid Poisoning (AZP) is a human toxic syndrome which is associated with the consumption of bivalve shellfish. Unlike other shellfish, mussels contain a large array of azaspiracid analogs, many of which are suspected bioconversion products. These studies were conducted to elucidate the metabolic pathways of azaspiracid (AZA1) in the blue mussel (Mytilus edulis) and revealed that the main biotransformation product was the more toxic demethyl analog, AZA3. To elucidate the mechanism of this C-demethylation, an unprecedented xenobiotic bioconversion step in shellfish, AZA1 was fed to mussels that contained no detectable azaspiracids. Triple quadrupole mass spectrometry (MS) and high resolution Orbitrap MS were used to determine the uptake of AZA1 and the toxin profiles in three tissue compartments of mussels. The second most abundant bioconversion product was identified as AZA17, a carboxyl analog of AZA3, which is a key intermediate in the formation of AZA3. Also, two pairs of isomeric hydroxyl analogs, AZA4/AZA5 and AZA7/AZA8, have been confirmed as bioconversion products for the first time. Ultra high resolution (100 k) MS studies showed that the most probable structural assignment for AZA17 is 22-carboxy-AZA3 and a mechanism for its facile decarboxylation to form AZA3 has been proposed.

Population pharmacokinetics meta-analysis of plitidepsin (Aplidin) in cancer subjects

OBJECTIVE

To characterize the population pharmacokinetics of plitidepsin (Aplidin) in cancer patients.

METHODS

A total of 283 patients (552 cycles) receiving intravenous plitidepsin as monotherapy at doses ranging from 0.13 to 8.0 mg/m(2) and given as 1- or 24-h infusions every week; 3- or 24-h infusion biweekly; or 1-h infusion daily for 5 consecutive days every 21 days were included in the analysis. An open three-compartment pharmacokinetic model and a nonlinear binding to red blood cells model were used to describe the plitidepsin pharmacokinetics in plasma and blood, respectively, using NONMEM V software. The effect of selected covariates on plitidepsin pharmacokinetics was investigated. Model evaluation was performed using goodness-of-fit plots, posterior predictive check and bootstrap.

RESULTS

Plasma clearance and its between subject variability (%) was 13.6 l/h (71). Volume of distribution at steady-state was calculated to be 4791 l (59). The parameters B (max) and C (50) of the non-linear blood distribution were 471 microg/l (56) and 41.6 microg/l, respectively. Within the range of covariates studied, age, sex, body size variables, aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), total bilirubin, creatinine clearance, albumin, total protein, performance status, co-administration of inhibitors or inducers of CYP3A4 and presence of liver metastases were not statistically related to plitidepsin pharmacokinetic parameters. Bootstrap and posterior predictive check evidenced the model was deemed appropriate to describe the time course of plitidepsin blood and plasma concentrations in cancer patients.

CONCLUSIONS

The integration of phase I/II pharmacokinetic data demonstrated plitidepsin linear elimination from plasma, dose-proportionality up to 8.0 mg/m(2), and time-independent pharmacokinetics. The distribution to red blood cells can be considered linear at doses lower than 5 mg/m(2) administered as 3-h or longer infusion. No clinically relevant covariates were identified as predictors of plitidepsin pharmacokinetics.

Synthesis and structure-activity relationship of cytotoxic marine cyclodepsipeptide IB-01212 analogues

Several recently discovered marine products have remarkable in vitro and in vivo anticancer profiles against a wide range of tumor cell lines. Some of these compounds are currently in clinical trials. These compounds show complex structures and mechanisms of action of interest. Herein, we describe the preparation of a series of totally synthetic molecules that are structurally related to the natural marine product IB-01212 and evaluated them as antitumor agents. For this, total solid-phase syntheses of the products were performed in parallel by two distinct routes: linear synthesis and convergent synthesis. Structural modifications were introduced in several residue positions to afford 21 IB-01212 analogues for structure-relationship studies. An increase in the number of methyl groups in the macrocycle enhanced cytotoxic activity. Also, the replacement of an ester bond by an amide bond favored antitumor activity against several human cell lines. In addition, the L configuration analogues were more active against all the tumor cell lines than those containing the D configuration. A significant increase in the size and asymmetry of the macrocycle diminished biological activity with respect to that of IB-01212. These results are of great value for the discovery of new and more effective anticancer agents.