Exploring the impact of CYP2D6 and UGT2B7 gene-drug interactions, and CYP-mediated DDI on oxycodone and oxymorphone pharmacokinetics using physiologically-based pharmacokinetic modeling and simulation.

Oxycodone is one of the most commonly used opioids to treat moderate to severe pain. It is metabolized mainly by CYP3A4 and CYP2D6, while only a small fraction of the dose is excreted unchanged into the urine. Oxymorphone, the metabolite primarily formed by CYP2D6, has a 40- to 60-fold higher mu-opioid receptor affinity than the parent compound. While CYP2D6-mediated gene-drug-interactions (GDIs) and drug-drug interactions (DDIs) are well-studied, they only account for a portion of the variability in oxycodone and oxymorphone exposure. The combined impact of CYP2D6-mediated GDIs and DDIs, CYP3A4-mediated DDIs, and UGT2B7 GDIs is not fully understood yet and hard to study in head-to-head clinical trials given the relatively large number of scenarios. Instead, we propose the use of a physiologically-based pharmacokinetic model that integrates available information on oxycodone's metabolism to characterize and predict the impact of DDIs and GDIs on the exposure of oxycodone and its major, pharmacologically-active metabolite oxymorphone. To this end, we first developed and verified a PBPK model for oxycodone and its metabolites using published clinical data. The verified model was then applied to determine the dose-exposure relationship of oxycodone and oxymorphone stratified by CYP2D6 and UGT2B7 phenotypes respectively, and administered perpetrators of CYP-based drug interactions. Our simulations demonstrate that the combination of CYP2D6 UM and a UGT2B7Y (268) mutation may lead to a 2.3-fold increase in oxymorphone exposure compared to individuals who are phenotyped as CYP2D6 NM / UGT2B7 NM. The extent of oxymorphone exposure increases up to 3.2-fold in individuals concurrently taking CYP3A4 inhibitors, such as ketoconazole. Inhibition of the CYP3A4 pathway results in a relative increase in the partial metabolic clearance of oxycodone to oxymorphone. Oxymorphone is impacted to a higher extent by GDIs and DDIs than oxycodone. We predict oxymorphone exposure to be highest in CYP2D6 UMs/UGT2B7 PMs in the presence of ketoconazole (strong CYP3A4 index inhibitor) and lowest in CYP2D6 PMs/UGT2B7 NMs in the presence of rifampicin (strong CYP3A4 index inducer) covering a 55-fold exposure range.

The Association of Serotonin Toxicity with Combination Linezolid-Serotonergic Agent Therapy: A Systematic Review and Meta-Analysis.

Linezolid (LZD) has a longstanding reported association with the onset of serotonin toxicity (ST), secondary to drug-drug interactions with serotoninergic agents. There have been no conclusive data supporting the incidence or contributing risk factors to date. The study evaluated the incidence of ST in patients treated with LZD and serotonergic agents concomitantly versus LZD alone. The secondary objectives included a comparison of ST incidence in patients treated with one serotonergic agent + LZD versus two or more serotonergic agents + LZD. The studies used for this meta-analysis were retrieved from PubMed, Scopus, and Google Scholar. All studies including a comparison between LZD alone and LZD + a serotonergic agent published between 1 January 2000 and 1 October 2023 and meeting the quality standards were considered for inclusion. Fourteen studies were identified, with five meeting all inclusion and exclusion criteria with no significant heterogeneity. For the analysis of LZD monotherapy vs. SA combination therapy, four studies with 6025 patients total were analyzed and yielded an odds ratio of 1.78 (CI [1.04, 3.02]; I2 = 49%; GRADE certainty: low). Four studies and 2501 patients were included in the analysis of one versus more than one SA with an odds ratio of 5.18 (CI [1.05, 25.49]; I2 = 44.87; GRADE certainty: moderate). The Newcastle-Ottawa score, visual inspection of the funnel plot, and Egger's statistic were used to evaluate quality and heterogeneity. The Peto method was used to calculate the summary odds ratios. All analyses were performed using Comprehensive Meta-Analysis version 3.0 and R, while GRADE was used to evaluate the quality of the final recommendation. The number of concomitant serotonergic agents may play a role in the development of serotonin toxicity in patients prescribed linezolid. In patients requiring linezolid therapy and serotonergic agents, risk versus benefit analysis should pay attention to the number of interacting drugs.

Clinical Impact of the CYP2C19 Gene on Diazepam for the Management of Alcohol Withdrawal Syndrome.

Diazepam is a benzodiazepine widely prescribed for the management of patients with severe alcohol withdrawal syndrome to prevent agitation, withdrawal seizures, and delirium tremens. Despite standard dosing of diazepam, a subset of patients experience refractory withdrawal syndromes or adverse drug reactions, such as impaired motor coordination, dizziness, and slurred speech. The CYP2C19 and CYP3A4 enzymes play a key role in the biotransformation of diazepam. Given the highly polymorphic nature of the CYP2C19 gene, we reviewed the clinical impact of variants in the CYP2C19 gene on both the pharmacokinetics of diazepam and treatment outcomes related to the management of alcohol withdrawal syndrome.

Pharmacogenetic Testing in a 70-Year-Old Woman with Polypharmacy and Multiple Comorbidities: A Case Report.

BACKGROUND Comorbidities and polypharmacy are difficult to manage, as polypharmacy hinders identification and prevention of medication-related problems. Risk for adverse drug events (ADEs) can be minimized through pharmacogenomic (PGx) testing and related therapeutic adjustments. CASE REPORT A 70-year-old woman with comorbidities and medications enrolled in the Program of All-inclusive Care for the Elderly presented with left lower extremity (LLE) pain, generalized weakness, and major depressive disorder. The provider requested a medication safety review, where the clinical pharmacist-recommended PGx testing given the LLE pain and weakness while taking a statin and inconsistent INR readings taking warfarin. The pharmacist recommended switching atorvastatin to pravastatin to minimize the risk for statin-associated ADEs due to CYP3A4 inhibition and switching fluoxetine to citalopram due to uncontrolled depression/anxiety and to mitigate drug-drug interactions with carvedilol to reduce the risk of orthostatic hypotension. Recommendations were accepted and upon follow-up the patient reported minor LLE pain and improved wellbeing on citalopram. Following PGx testing, the patient had decreased function at SLCO1B1 and was an intermediate metabolizer for CYP2C9 and CYP2D6. This case demonstrates how preemptive PGx testing would have identified drug-gene interactions (DGIs) at the time of prescribing and reduced the risk of statin-associated muscular symptoms, highlighting the utility of panel-based PGx testing in older adults at high risk for ADEs and/or therapy failure. CONCLUSIONS Decreased function at SLCO1B1 increases exposure to statins, leading to statin-induced myalgias, as displayed in this case. PGx testing can help identify DGIs, choose optimal therapies in medically complex older adults, and minimize ADE risk.

Multidrug Interactions: Why Do They Occur and How to Handle?

A nonoptimized medication therapy (NOMT) event is an iatrogenic hazard or incident associated with medications and is a leading cause of death, serious injury, and illness. NOMT events are often related to multidrug interactions in patients with polypharmacy. In these patients, NOMT events can be avoided by using advanced clinical decision support systems and clinical interventions such as separating the time of administration of certain drugs during the day. At the individual level, medication reconciliation is a first logical step for reducing adverse side effects. Then, intersubject variability in drug response should be considered to optimize patient drug regimens. Furthermore, patient pharmacogenomic status information can help ensure appropriateness of drug therapy. However, in patients with polypharmacy, such information is most valuable when combined with phenoconversion probability. At a population level, the virtual addition of drugs to various drug regimens and the use of a medication risk score can help predict the risk of NOMT events. This review outlines some of the mechanisms behind multidrug interactions and their association with drug safety and NOMTs, polypharmacy and its impact on patient outcomes, the value of pharmacogenomics, and an assessment of simulation studies and the virtual addition of drugs to a drug regimen using real-world data.

Case Report: Performing a Medication Safety Review Assisted by Pharmacogenomics to Explain a Prescribing Cascade Resulting in a Patient Fall.

Pharmacotherapy for major depressive disorder (MDD) typically consists of trial-and-error and clinician preference approaches, where patients often fail one or more antidepressants before finding an optimal regimen. Pharmacogenomics (PGx) can assist in prescribing appropriate antidepressants, thereby reducing the time to MDD remission and occurrence of adverse drug events. Since many antidepressants are metabolized by and/or inhibit cytochrome P450 enzymes (e.g., CYP2C19 or CYP2D6), drug-induced phenoconversion is common in patients on antidepressant combinations. This condition influences the interpretation of a patient's PGx results, overall risk of ineffective/adverse medication response due to multi-drug interactions, and the recommendations. This complex case describes a patient with MDD, generalized anxiety disorder, and chronic pain who experienced a fall due to excessive sedation following a prescribing cascade of fluoxetine, bupropion, and doxepin. These antidepressants delivered a significant additive sedative effect and interacted with the patient's hydrocodone, potentially contributing to uncontrolled pain, upward dose titration of hydrocodone, and a higher overall sedative burden. The PGx results and drug-induced phenoconversion described in this case report explain the patient's excessive sedation and possibly ineffective/toxic antidepressant and opioid treatment. This case report also illustrates how a more timely multi-drug interaction assessment (preferably in conjunction with preemptive PGx testing) may have informed a different prescribing pattern, reduced/avoided a prescribing cascade, and potentially prevented a drug-related fall.

Implementing pharmacists' medication safety recommendations is associated with reduced health care resource utilization and mortality in high-risk older Americans.

BACKGROUND: A medication safety review (MSR) is a novel, pharmacist-driven, technology-supported intervention that prioritizes adverse drug event risk mitigation. Previous research has shown that Medicare Part D beneficiaries who received MSRs in an enhanced medication therapy management (EMTM) model realized improvements in total Medicare spending, hospitalizations, emergency department (ED) visits, and mortality compared to control. However, it is unknown whether beneficiaries implemented pharmacists' MSR recommendations. OBJECTIVE: The objective of the study was to evaluate whether MSR recommendation implementation is associated with improvements in these same outcomes for Part D beneficiaries enrolled in EMTM compared to a control group. METHODS: This retrospective, pre-post, cohort study evaluated outcomes for beneficiaries who were targeted for MSR services in 2018 and 2019. The "validated implementation MSR (viMSR)" cohort included those who received their first-ever MSR in 2018, received another MSR in 2019, and validated implementation of ≥1 recommendation in their 2018 MSR. The "failed to engage" (FTE) cohort included beneficiaries who were targeted for MSR services in both 2018 and 2019 but did not engage in an MSR at any point through the end of 2019. For both cohorts, we calculated the 2018-to-2019 change for each outcome and then determined whether year-over-year changes differed significantly between cohorts. For mortality, we relaxed the requirement for continuous enrollment in 2019, permitting us to compare the proportion of beneficiaries that died in each group in 2019. Analyses were adjusted for baseline multimorbidity. RESULTS: Of 4384 beneficiaries who completed MSRs, 602 (13.7%) implemented ≥1 recommendation. The viMSR cohort (N = 602) outperformed the FTE cohort (N = 7052) in total Medicare costs ($2162/y lower; P = 0.020), Part A Medicare costs ($1855/y; P = 0.024), hospitalizations (9.1 fewer admissions/100 beneficiaries/y, P = 0.020), ED visits (10.8 fewer visits/100 beneficiaries/y, P = 0.014), and mortality (3.8% fewer died in 2019; P < 0.001). CONCLUSION: Implementing pharmacists' recommendations in MSRs was associated with improved health care resource utilization and mortality for MSR-eligible beneficiaries.

Utilizing Pharmacogenomics Results to Determine Opioid Appropriateness and Improve Pain Management in a Patient with Osteoarthritis.

The opioid epidemic in the United States has exposed the need for providers to limit opioid dispensing and identify at-risk patients prior to prescribing opioids. With pharmacogenomic testing, clinicians can analyze hundreds of medications-including commonly prescribed opioids-against genetic results to understand and predict risk and response. Moreover, knowledge of genotypic variants and altered function can help decrease trial and error prescribing, identify patients at-risk for adverse drug events, and improve pain control. This patient case demonstrates how pharmacogenomic test results identified drug-gene interactions and provided insight about a patient's inadequate opioid therapy response. With pharmacogenomic information, the patient's healthcare team discontinued opioid therapy and selected a more appropriate regimen for osteoarthritis (ie, celecoxib), resulting in improved pain control and quality of life.

Pharmacist-driven interventions to de-escalate urinary antimuscarinics in the Programs of All-Inclusive Care for the Elderly.

BACKGROUND: Given associations with serious cognitive and physical adverse effects (e.g., dementia, falls), strong anticholinergics, like urinary antimuscarinics (UAMs), should be avoided in older adults. This feasibility study aimed to (1) evaluate the implementation rate of pharmacists' recommendations intended to de-escalate UAMs, (2) quantify the change in overall anticholinergic dosing exposure from these recommendations, and (3) investigate factors that predict recommendation implementation. METHODS: This was a retrospective, observational, before-and-after study. Pharmacists (n = 18) devised strategies to de-escalate UAMs in 187 participants (mean age 72.4 ± 9.4; 77.0% female; mean number of medications 12.9 ± 4.6) of 35 Programs of All-Inclusive Care for the Elderly (PACE). PACE prescribers (non-physicians and physicians) determined whether to implement recommendations. Implementation was defined as a change in the prescription records consistent with the pharmacist's recommendation at 2-, 4-, 6-, and 9-months post-recommendation. Anticholinergic dosing exposure was measured at each time point using standardized daily doses (SDD). Multivariable logistic regression was used to identify factors that predicted recommendation implementation. RESULTS: Across 9 months, recommendations were implemented in 118 out of 187 participants, yielding a 63.1% implementation rate. Of these, 77.1% (n = 91/118) implemented by month 2. Implementers' mean overall anticholinergic SDD decreased 65.4% from baseline (baseline: 2.6 [95% CI: 2.2, 3.0] to month 9: 0.9 [95% CI: 0.6,1.2], p < 0.001) whereas non-implementers demonstrated no significant change (p = 0.52). Taking <10 baseline medications (OR 2.75; 95% CI: 1.09, 7.61); baseline UAM SDD ≥2 (OR 2.20; 95% CI: 1.11, 4.44); uncomplicated recommendations (OR 3.38; 95% CI: 1.67-7.03); and baseline calcium channel blocker use (OR 2.19; 95% CI: 1.09, 4.52) predicted implementation. CONCLUSION: Our high implementation rate indicates that pharmacists' recommendations to de-escalate UAMs as a way to reduce overall anticholinergic exposure is feasible in medically complex, community-dwelling older adults. Future research should investigate whether these recommendations benefit cognitive (e.g., delirium, dementia) and/or physical functioning (e.g., falls).

Pharmacogenomics and Drug-Induced Phenoconversion Informed Medication Safety Review in the Management of Pain Control and Quality of Life: A Case Report.

Utilizing pharmacogenomics (PGx) and integrating drug-induced phenoconversion to guide opioid therapies could improve the treatment response and decrease the occurrence of adverse drug events. Genetics contribute to the interindividual differences in opioid response. The purpose of this case report highlights the impact of a PGx-informed medication safety review, assisted by a clinical decision support system, in mitigating the drug-gene and drug-drug-gene interactions (DGI and DDGI, respectively) that increase the risk of an inadequate drug response and adverse drug events (ADEs). This case describes a 69-year-old female who was referred for PGx testing for uncontrolled chronic pain caused by osteoarthritis and neuropathy. The clinical pharmacist reviewed the PGx test results and medication regimen and identified several (DGIs and DDGIs, respectively) at Cytochrome P450 (CYP) 2C19 and CYP2D6. The recommendations were to: (1) switch tramadol to buprenorphine transdermal patch, an opioid with lower potential for ADEs, to mitigate a CYP2D6 DDGI; (2) gradually discontinue amitriptyline to alleviate the risk of anticholinergic side effects, ADEs, and multiple DDGIs; and (3) optimize the pregabalin. The provider and the patient agreed to implement these recommendations. Upon follow-up one month later, the patient reported an improved quality of life and pain control. Following the amitriptyline taper, the patient experienced tremors in the upper and lower extremities. When the perpetrator drug, omeprazole, was stopped, the metabolic capacity was no longer impeded; the patient experienced possible amitriptyline withdrawal symptoms due to the rapid withdrawal of amitriptyline, which was reinitiated and tapered off more slowly. This case report demonstrates a successful PGx-informed medication safety review that considered drug-induced phenoconversion and mitigated the risks of pharmacotherapy failure, ADEs, and opioid misuse.

Revisiting conclusions on the empiric segmentation of high-risk patients.

The authors provide feedback on generalizations made regarding interventions for high-risk populations in previous research.

Assessing the Impact of an Advanced Clinical Decision Support System on Medication Safety and Hospital Readmissions in an Innovative Transitional Care Model: A Pilot Study.

(1) Background: Adverse drug events and inappropriate use of medications lead to hospitalizations, medication-related morbidity, and mortality. This study examined whether a novel medication risk prediction tool, the MedWise Risk Score™, was associated with medication safety-related problem (MRP) identification and whether integration into an existing innovative transitions of care (TOC) service could decrease readmissions. (2) Methods: This retrospective comparator group study assessed patients discharged from a hospital in southern Arizona between January and December 2020. Participants were included in the study if they were 18 years of age or older, referred to the pharmacist for TOC services, and received a pharmacist consultation within one-week post discharge. Patients were categorized into two groups: (1) medication safety review (MSR)-TOC service (intervention) or (2) existing innovative TOC service (control). (3) Results: Of 164 participants, most were male (57%) and were between 70−79 years of age. Overall, there were significantly more drug-drug interactions (DDI) MRPs identified per patient in the intervention vs. control group for those who were readmitted (3.7 ± 1.5 vs. 0.9 ± 0.6, p < 0.001) and those who were not readmitted (2 ± 1.3 vs. 1.3 ± 1.2, p = 0.0120). Furthermore, of those who were readmitted, the average number of identified MRPs per patient was greater in the intervention group compared to the control (6.3 vs. 2.5, respectively, p > 0.05). Relative to the control, the readmission frequency was 30% lower in the treatment group; however, there was insufficient power to detect significant differences between groups. (4) Conclusions: The integration of a medication risk prediction tool into this existing TOC service identified more DDI MRPs compared to the previous innovative TOC service, which lends evidence that supports its ability to prevent readmissions. Future work is warranted to demonstrate the longitudinal impact of this intervention in a larger sample size.

Evaluating the Impact of Medication Risk Mitigation Services in Medically Complex Older Adults.

Adverse drug events (ADEs) represent an expensive societal burden that disproportionally affects older adults. Therefore, value-based organizations that provide care to older adults­such as the Program of All-Inclusive Care for the Elderly (PACE)­should be highly motivated to identify actual or potential ADEs to mitigate risks and avoid downstream costs. We sought to determine whether PACE participants receiving medication risk mitigation (MRM) services exhibit improvements in total healthcare costs and other outcomes compared to participants not receiving structured MRM. Data from 2545 PACE participants from 19 centers were obtained for the years 2018 and 2019. We compared the year-over-year changes in outcomes between patients not receiving (control) or receiving structured MRM services. Data were adjusted based on participant multimorbidity and geographic location. Our analyses demonstrate that costs in the MRM cohort exhibited a significantly smaller year-to-year increase compared to the control (MRM: USD 4386/participant/year [95% CI, USD 3040−5732] vs. no MRM: USD 9410/participant/year [95% CI, USD 7737−11,084]). Therefore, receipt of structured MRM services reduced total healthcare costs (p < 0.001) by USD 5024 per participant from 2018 to 2019. The large majority (75.8%) of the reduction involved facility-related expenditures (e.g., hospital admission, emergency department visits, skilled nursing). In sum, our findings suggest that structured MRM services can curb growing year-over-year healthcare costs for PACE participants.

Mitigating Benzodiazepine Dependence and the Risk of Drug-Induced QTc Prolongation in the Treatment of Gastroparesis: A Case Report.

Patients are often faced with challenges when it comes to safe therapeutic options. An 89-year-old female with a history of arrhythmias and refractory gastroparesis complained of adverse drug events from her benzodiazepine. While performing a comprehensive medication review and a medication safety review using an advanced clinical decision support system, the pharmacist successfully tapered off the benzodiazepine to a safer alternative antidepressant indicated for the treatment of gastroparesis. Special attention was given to selecting drugs with less QT prolongation risk, based on her age, current drug regimen, previous medical history, and presence of polypharmacy.

Mitigating the Risk of Adverse Effects Related to Augmentation Therapy for Resistant Major Depressive Disorder: A Case Report.

Polypharmacy of psychotropic medications predisposes older adults to adverse drug events (ADEs). One contributing factor is inhibition of metabolic pathways between substrates (competitive inhibition) or between substrates and inhibitors of the same cytochrome P450 (CYP450) isoforms. The purpose of this case report is to demonstrate observed sedation and difficulty concentrating from augmentation therapy for resistant major depressive disorder (MDD) and to highlight the value of clinical tools to identify opportunities for treatment optimization to reduce ADEs. The pharmacist identified significant medication burden and competitive inhibition of drug metabolism in the CYP450 system during a telehealth medication therapy management consultation with a 69-year-old male. The pharmacist recommended clinical monitoring and communicated concerns about medication-induced sedation, difficulty concentrating, and other medication-related problems (MRP) to providers. Several recommendations were implemented which helped improved patient's outcomes. Individualizing MDD pharmacotherapy based on pharmacokinetic and pharmacodynamic drug interactions and geriatric dosage considerations may lead to better outcomes and tolerability among older adults.

Physiologically Based Pharmacokinetic Modeling to Assess the Impact of CYP2D6-Mediated Drug-Drug Interactions on Tramadol and O-Desmethyltramadol Exposures via Allosteric and Competitive Inhibition.

Tramadol is an opioid medication used to treat moderately severe pain. Cytochrome P450 (CYP) 2D6 inhibition could be important for tramadol, as it decreases the formation of its pharmacologically active metabolite, O-desmethyltramadol, potentially resulting in increased opioid use and misuse. The objective of this study was to evaluate the impact of allosteric and competitive CYP2D6 inhibition on tramadol and O-desmethyltramadol pharmacokinetics using quinidine and metoprolol as prototypical perpetrator drugs. A physiologically based pharmacokinetic model for tramadol and O-desmethyltramadol was developed and verified in PK-Sim version 8 and linked to respective models of quinidine and metoprolol to evaluate the impact of allosteric and competitive CYP2D6 inhibition on tramadol and O-desmethyltramadol exposure. Our results show that there is a differentiated impact of CYP2D6 inhibitors on tramadol and O-desmethyltramadol based on their mechanisms of inhibition. Following allosteric inhibition by a single dose of quinidine, the exposure of both tramadol (51% increase) and O-desmethyltramadol (52% decrease) was predicted to be significantly altered after concomitant administration of a single dose of tramadol. Following multiple-dose administration of tramadol and a single-dose or multiple-dose administration of quinidine, the inhibitory effect of quinidine was predicted to be long (≈42 hours) and to alter exposure of tramadol and O-desmethyltramadol by up to 60%, suggesting that coadministration of quinidine and tramadol should be avoided clinically. In comparison, there is no predicted significant impact of metoprolol on tramadol and O-desmethyltramadol exposure. In fact, tramadol is predicted to act as a CYP2D6 perpetrator and increase metoprolol exposure, which may necessitate the need for dose separation.

Emerging Non-statin Treatment Options for Lowering Low-Density Lipoprotein Cholesterol.

Low-density lipoprotein cholesterol (LDL-C) is a modifiable risk factor for the development of atherosclerotic cardiovascular disease. Statins have been the gold standard for managing cholesterol levels and reducing the risks associated with atherosclerotic cardiovascular disease; however, many patients do not achieve their cholesterol goals or are unable to tolerate this drug class due to adverse drug events. Recent studies of non-statin cholesterol lowering drugs (i.e., ezetimibe, PCSK9 inhibitors) have demonstrated cardiovascular benefits; and new drugs [i.e., bempedoic acid (BDA), inclisiran] have produced promising results in pre-clinical and clinical outcome trials. This narrative review aims to discuss the place in therapy of ezetimibe, PCSK9 inhibitors, BDA, and inclisiran and describe their relative pharmacokinetic (PK) profiles, efficacy and safety as monotherapy and combination therapy, and cardiovascular benefit(s) when used for hypercholesterolemia.

Determination of CYP450 Expression Levels in the Human Small Intestine by Mass Spectrometry-Based Targeted Proteomics.

The human small intestine can be involved in the first-pass metabolism of drugs. Under this condition, members of the CYP450 superfamily are expected to contribute to drug presystemic biotransformation. The aim of this study was to quantify protein expression levels of 16 major CYP450 isoforms in tissue obtained from nine human organ donors in seven subsections of the small intestine, i.e., duodenum (one section, N = 7 tissue samples), jejunum (three subsections (proximal, mid and distal), N = 9 tissue samples) and ileum (three subsections, (proximal, mid and distal), N = 9 tissue samples), using liquid chromatography tandem mass spectrometry (LC-MS/MS) based targeted proteomics. CYP450 absolute protein expression levels were compared to mRNA levels and enzyme activities by using established probe drugs. Proteins corresponding to seven of sixteen potential CYP450 isoforms were detected and quantified in various sections of the small intestine: CYP2C9, CYP2C19, CYP2D6, CYP2J2, CYP3A4, CYP3A5 and CYP4F2. Wide inter-subject variability was observed, especially for CYP2D6. CYP2C9 (p = 0.004) and CYP2C19 (p = 0.005) expression levels decreased along the small intestine. From the duodenum to the ileum, CYP2J2 (p = 0.001) increased, and a trend was observed for CYP3A5 (p = 0.13). CYP3A4 expression was higher in the jejunum than in the ileum (p = 0.03), while CYP4F2 expression was lower in the duodenum compared to the jejunum and the ileum (p = 0.005). CYP450 protein levels were better correlated with specific isoform activities than with mRNA levels. This study provides new data on absolute CYP450 quantification in human small intestine that could improve physiologically based pharmacokinetic models. These data could better inform drug absorption profiles while considering the regional expression of CYP450 isoforms.