The review, authored by Shivam Kumar Vyas, Md Imran Ahmed, and Vaibhav A. Dixit at the National Institute of Pharmaceutical Education and Research in Guwahati, India, takes on the question embedded in its title: are cytochrome P450 inhibitors friends or foes? The authors’ answer is deliberately nuanced. Inhibition of specific CYP450 isoforms has emerged as a powerful therapeutic strategy in cancer and infectious disease, while unintended inhibition of drug-metabolizing isoforms remains a leading cause of clinically significant interactions. The challenge, they conclude, is not whether CYP450 inhibition is good or bad, but whether scientists can reliably predict which outcome a given inhibitor will produce.

The therapeutic case for inhibition is striking. CYP19A1, also known as aromatase, catalyzes the conversion of androgens to estrogens, and its blockade by drugs such as letrozole and anastrozole starves hormone-dependent breast cancer cells of the estrogen they need to proliferate. CYP17A1 plays a critical role in androgen biosynthesis, and its inhibition by abiraterone suppresses androgen production in prostate cancer. In the endocrine arena, inhibitors of CYP11B1 and CYP11B2, which drive corticosteroid and aldosterone synthesis, have been explored for Cushing’s syndrome and hypertension. Perhaps most famously, fungal CYP51 catalyzes a key demethylation step in ergosterol biosynthesis, and azole antifungals such as ketoconazole, itraconazole, and fluconazole exploit this dependency to collapse fungal cell membranes. Pathogen CYP450 enzymes have even been pursued as targets in tuberculosis and leishmaniasis.

The same chemistry, however, can turn hostile. Ketoconazole’s inhibition of CYP3A4 dramatically raises plasma levels of drugs like midazolam and simvastatin, producing excessive sedation or, in the case of statins, rhabdomyolysis risk. Fluconazole’s inhibition of CYP2C9 can elevate warfarin concentrations and precipitate bleeding complications. Prodrugs present a subtler trap: clopidogrel requires CYP2C19-mediated activation, and co-administration with inhibitors such as omeprazole blunts the formation of its active metabolite, diminishing antiplatelet efficacy and raising cardiovascular risk. Whether inhibition helps or harms, the authors argue, depends on isoform selectivity, the mechanism of inhibition, and — crucially — what metabolism actually does to the drug in question.

To bring order to a field crowded with overlapping terminology, the review proposes a unified classification. CYP450 inhibitors are sorted into two major mechanistic categories: reversible inhibitors and mechanism-based, or time-dependent, inhibitors. Reversible inhibitors, which are typically competitive and display Type I spectroscopic signatures, occupy the active site without undergoing oxidation. Mechanism-based inhibitors are subdivided into quasi-irreversible and irreversible types. Quasi-irreversible inhibitors coordinate the heme iron through a heterocyclic nitrogen or form metabolite-intermediate complexes, as seen with miconazole, paroxetine, and tadalafil. Irreversible inhibitors, by contrast, are bioactivated by the enzyme itself into reactive metabolites that covalently modify the heme or protein residues — a process the authors prefer to describe without the popular but imprecise label of suicide inhibition.

A key analytical contribution is the authors’ reassessment of kinetic behavior using a modified Michaelis-Menten parameter, β = Km/Ki, which captures the balance between a molecule’s role as a substrate and as an inhibitor of the same enzyme. When log β exceeds 1, non-productive inhibitory binding dominates and the enzyme effectively self-inhibits; when log β falls below 1, productive metabolism prevails. Applying this framework, phenacetin at CYP1A2 shows a log β of roughly 2.64, marking it as strongly inhibitor-like, while dextromethorphan at CYP2D6, with a log β near 0.19, behaves predominantly as a substrate. For mechanism-based inhibitors, the authors introduce a partitioning ratio, kinact/koff, derived from measurable equilibrium constants rather than hard-to-obtain rate constants, to quantify the relative contributions of reversible and irreversible binding. Miconazole’s ratio of 14 signals rapid irreversible binding, whereas tadalafil’s near-zero ratio indicates balanced contributions from both modes.

Spectroscopy provides an independent lens on these mechanisms. The resting ferric enzyme absorbs near 417–420 nanometers in its low-spin state. Type I ligands, which are substrate-like and displace the axial water ligand, shift the heme iron to a high-spin configuration and produce a characteristic difference spectrum with a peak near 385–390 nanometers and a trough near 418–425 nanometers. Type II ligands, typically nitrogen heterocycles such as imidazoles and triazoles, coordinate directly to the ferric heme iron, stabilize the low-spin state, and generate a red-shifted signature with a crest between 425 and 435 nanometers. Yet the authors caution that spectroscopic class and kinetic mechanism do not always align: ketoconazole, though classically Type II, exhibits mixed inhibition kinetics at CYP3A4 because the enzyme’s large, flexible active site can accommodate multiple ligands simultaneously.

On the regulatory side, the review walks through the FDA’s in vitro guidance for evaluating CYP450 substrates, inhibitors, and inducers, and proposes a streamlined workflow that integrates reversible and time-dependent inhibition testing into a single protocol. Reaction phenotyping with recombinant enzymes identifies which isoforms handle a compound; if a single enzyme accounts for more than 25 percent of metabolism, the compound is flagged as a sensitive substrate warranting further study. Inhibition screening at 10 to 30 micromolar, followed by IC50 determination and a preincubation-based IC50 shift assay, distinguishes reversible from time-dependent inhibitors. For reversible inhibitors, an R1 value below 1.02 clears the compound from further DDI concern; for time-dependent inhibitors, an R2 value below 1.25 serves the same gatekeeping function. The authors estimate their consolidated workflow could save four experimental steps compared with running the FDA’s separate protocols in parallel.

Perhaps the most practically useful contribution is a pair of decision criteria for predicting whether a given inhibitor will behave as a friend or a foe. Isoform selectivity, expressed as the difference in pIC50 between the therapeutic target and the nearest drug-metabolizing off-target, emerges as a strong predictor: a gap greater than one log unit — roughly tenfold selectivity — generally correlates with a favorable clinical profile. Letrozole, with a ΔpIC50 of 2.44, and ketoconazole, at 2.63, fit this pattern, while abiraterone, voriconazole, fluconazole, and itraconazole, with gaps at or below 0.75, are associated with significant clinical interactions. The second criterion concerns the direction of metabolism’s effect on drug action: when metabolism activates a prodrug or clears an active drug, inhibition tends to be harmful, as with ticagrelor plus ketoconazole or clopidogrel plus omeprazole; when metabolism inactivates the parent drug, inhibition can be harnessed as a pharmacokinetic booster, as ritonavir and cobicistat do for darunavir, atazanavir, lopinavir, and nirmatrelvir. Ritonavir itself illustrates the context-dependence, acting as a friend for some partners and a foe for others, including posaconazole and venetoclax.

The review closes with a survey of computational models developed over the past five years to predict CYP450 inhibition, ranging from random-forest classifiers like CYPlebrity to graph convolutional networks, deep neural networks such as CYPi-DNN, and multimodal architectures like the Multimodal Encoding Network, which combines molecular fingerprints, graphs, and protein sequences to achieve a reported 93.7 percent accuracy and 98.5 percent AUC. Yet the authors identify a conspicuous gap: no existing model directly predicts isoform selectivity, the very property their analysis shows is most predictive of clinical friend-or-foe behavior. Combining independently trained isoform-specific models, they note, is not a valid substitute because such combinations require their own validation. Until that gap is closed, the review’s central message stands: CYP450 inhibition is neither inherently friend nor foe, and the tools to tell the difference — kinetic, spectroscopic, and now computational — are finally converging on a coherent framework for making that call rationally.

Subject of Research: Mechanisms, classification, and therapeutic roles of cytochrome P450 enzyme inhibitors in drug metabolism and drug-drug interactions

Article Title: Cytochrome P450 inhibitors: friends or foes?

Article References: Vyas, S. K., Ahmed, M. I., & Dixit, V. A. (2026). Cytochrome P450 inhibitors: friends or foes?. Frontiers in Chemistry, 14, Article 1988344. https://doi.org/10.3389/fchem.2026.1988344

Image Credits: AI Generated

DOI: 10.3389/fchem.2026.1988344

Keywords: cytochrome P450, drug metabolism, drug-drug interactions, enzyme inhibition, pharmacokinetics, mechanism-based inhibition, CYP3A4, azole antifungals, machine learning, drug discovery, Type I and Type II ligands, pharmacokinetic boosting

The significance of the study lies partly in the sheer ubiquity of the exposure. Fungal infections are among the most frequent ailments in pregnancy, driven by hormonal and immunological changes that alter the body’s microbial balance. They are typically treated with azole antifungals, a broad family of compounds that includes both prescription drugs and, crucially, over-the-counter topical products such as creams and vaginal tablets. As Gylli Mola, a researcher and medical doctor at Rigshospitalet, points out, this is what makes the result particularly noteworthy: the treatments in question were primarily non-prescription topical preparations, which have traditionally been regarded as safe to use during pregnancy. If even these low-intensity, locally applied formulations are associated with hormonal signals in offspring, the assumption of safety may need to be re-examined with more rigorous data than currently exists.

The study rested on an unusually broad evidentiary base. The researchers analysed data from 589 Danish mother–child pairs, drawing on Denmark’s rich national registers and biobanks to compare children whose mothers had used azole antifungals during pregnancy with those who had not. Crucially, the epidemiological signal did not stand alone. The team supplemented the population data with laboratory experiments using human fetal adrenal tissue, allowing them to observe directly how azole compounds interact with the steroid-producing machinery of the gland. They also carried out studies in pregnant rats, examining both blood and adrenal tissue from the fetuses to see whether the same hormonal disruptions appeared in a controlled experimental setting. According to Monica Kam Draskau, a researcher at the DTU National Food Institute who worked on the experimental analyses, the fact that the findings in children are supported by results from both human fetal adrenal tissue and rat experiments strengthens the hypothesis that there is a genuine link with the use of azole-based antifungal medicines.

To understand why the adrenal gland matters here, it helps to consider what azoles actually do. Azole antifungals work by inhibiting enzymes in the cytochrome P450 family that fungi need to build ergosterol, an essential component of their cell membranes. The trouble is that humans rely on closely related P450 enzymes to manufacture steroid hormones, including the cortisol and other adrenal steroids that the fetus begins producing as its adrenal gland develops. A compound designed to block fungal steroid synthesis can, in principle, also interfere with human steroidogenesis, and the fetal adrenal gland is an extraordinarily active steroid-producing organ during gestation. The new findings are consistent with that mechanism: prenatal exposure to azoles appears to disrupt fetal steroidogenesis, and the disruption seems to leave traces that persist at least into early infancy.

Adrenal hormones are far from peripheral players in human physiology. They help regulate many functions in the body, including glucose metabolism, blood pressure, immune function, and growth and reproductive development. The adrenal gland also undergoes a remarkable transformation around the time of birth, known as the zona reticularis transition, when the fetal zones of the gland regress and adult-type steroid production takes over. Hormonal disturbances during this window could, in theory, ripple across multiple organ systems as a child grows. That is precisely why the Danish team is cautious but concerned: the hormone levels they measured at three months of age were still within the normal range, yet the pattern of change suggests that fetal development may have been affected in ways that standard clinical thresholds do not capture.

Professor Terje Svingen from the DTU National Food Institute, who led the work, described the persistence of the signal as striking. It is one thing, he noted, for a medicine to produce a transient hormonal blip that resolves within days; it is quite another for the use of azole-based antifungals during pregnancy to appear to affect adrenal hormone levels in children long after birth. The three-month timepoint means the differences survived the profound hormonal upheaval of delivery and the establishment of infant adrenal function. Svingen’s framing captures the careful middle ground the study occupies: the results suggest that fetal development may have been affected, but they stop well short of demonstrating harm, and the researchers are explicit that they do not know whether the differences will affect the children’s health or their ability to have children later in life.

That caution is scientifically essential. The study is observational in its human component, and it cannot directly establish that azole-based antifungal medicines cause the hormonal changes observed in children. Mothers who used antifungals may differ in other ways from those who did not, from infection severity to other medication use, and no observational design can fully exclude such confounding. Nor can the researchers conclude from this study that commonly used antifungal medicines are harmful to children. What the results do is raise a hypothesis, supported by converging lines of evidence, that now needs to be tested with dedicated studies. The authors argue that the safety of widely used antifungals during pregnancy warrants further investigation, and the triangulation of human data, human tissue experiments and animal studies gives that argument unusual weight for a finding of this kind.

The research was carried out as part of the MERLON research project and was funded by the Danish Environmental Protection Agency through the Danish Centre on Endocrine Disrupters, with additional support from the Rigshospitalet Research Council. IRSET in France was a partner in the study. That funding provenance is telling: endocrine disruption, the interference of chemicals with the body’s hormone systems, has become a major focus of European regulatory science, and medicines are increasingly scrutinised alongside industrial chemicals for their potential to perturb hormonal development. The authors declare no conflicts of interest, and the study appeared under the title ‘Prenatal antifungal exposure disrupts fetal steroidogenesis and is associated with persistent effects in children’.

For pregnant women, the immediate practical message is one of reassurance rather than alarm. Untreated fungal infections carry their own risks, and no current evidence indicates that a course of azole treatment harms a child; the measured hormone levels in the study remained within normal limits. The researchers themselves stress that they cannot conclude the medicines are harmful. The appropriate response, they suggest, is not for women to avoid needed treatment but for regulators, clinicians and researchers to generate the data needed to confirm or refute the signal. Larger cohorts, longer follow-up of exposed children, and mechanistic work clarifying which azoles, doses and exposure windows matter most would all help determine whether the hormonal differences observed at three months of age are a transient footprint or a marker of lasting developmental change.

The study also speaks to a broader lesson in pharmacology and public health: medicines assumed to be safe because they are old, familiar and available without prescription are not automatically free of subtle developmental effects. Topical preparations deliver lower systemic doses than oral drugs, but pregnancy is a period of exquisite hormonal sensitivity, and the fetal adrenal gland is one of the most hormonally active tissues in the body. The Danish findings do not overturn clinical practice, but they add an important data point to the ongoing effort to make medication use in pregnancy evidence-based rather than assumption-based. Until that evidence matures, the study stands as a carefully hedged but consequential reminder that even the most routine treatments of pregnancy deserve continued scientific scrutiny.

Subject of Research: Effects of prenatal azole antifungal exposure on fetal steroidogenesis and infant adrenal hormone levels

Article Title: New study raises questions about antifungal treatments during pregnancy

Article References: New study raises questions about antifungal treatments during pregnancy. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: azole antifungals, pregnancy, fetal development, adrenal hormones, steroidogenesis, endocrine disruption, Communications Medicine, DTU National Food Institute, Rigshospitalet, over-the-counter medicines, child health, pharmacovigilance