Gemfibrozil May Decrease Norclozapine Elimination: A Case Report

Exploring low clozapine C/D ratios, inverted clozapine-norclozapine ratios and undetectable concentrations as measures of non-adherence in clozapine patients: A literature review and a case series of 17 patients from 3 studies

BACKGROUND

Up to 1/2 of outpatients prescribed clozapine may be partially/fully non-adherent, based on therapeutic drug monitoring (TDM). Three indices for measuring partial/full non-adherence are proposed a: 1) clozapine concentration/dose (C/D) ratio which drops to half or more of what is expected in the patient; 2) clozapine/norclozapine ratio that becomes inverted; and 3) clozapine concentration that becomes non-detectable.

METHODS

These 3 proposed indices are based on a literature review and 17 cases of possible non-adherence from 3 samples: 1) an inpatient study in a Chinese hospital, 2) an inpatient randomized clinical trial in a United States hospital, and 3) and a Uruguayan outpatient study.

RESULTS

The first index of non-adherence is a clozapine C/D ratio which is less than half the ratio corresponding to the patient's specific ancestry group and sex-smoking subgroup. Knowing the minimum therapeutic dose of the patient based on repeated TDM makes it much easier to establish non-adherence. The second index is inverted clozapine/norclozapine ratios in the absence of alternative explanations. The third index is undetectable concentrations. By using half-lives, the chronology of the 3 indices of non-adherence was modeled in two patients: 1) the clozapine C/D ratio dropped to ≥1/2 of what is expected from the patient (around day 2); 2) the clozapine/norclozapine ratio became inverted (around day 3); and 3) the clozapine concentration became undetectable by the laboratory (around days 9-11).

CONCLUSION

Prospective studies should further explore these proposed clozapine indices in average patients, poor metabolizers (3 presented) and ultrarapid metabolizers (2 presented).

Making sense of norclozapine levels: 3 clinical axioms

Laboratories commonly provide norclozapine concentrations when a plasma clozapine level is requested, but the appropriate use of this information for the treatment of individuals with schizophrenia is not always clear. Particularly vexing is the fact that norclozapine possesses pharmacological properties that are distinct from its parent compound and which contribute to clozapine's efficacy signal, yet the literature focuses primarily on the association of clozapine levels with symptomatic improvement. The purpose of this brief article is to highlight findings with respect to the need to track norclozapine levels, or the ratio of clozapine/norclozapine plasma levels, to optimize efficacy among inadequate responders to clozapine treatment. In addition, there will be a discussion of the specific type of information provided by the clozapine/norclozapine ratio on clozapine's clearance, and how this ratio is sometimes misinterpreted. There is clinical value from to be derived from norclozapine levels and the clozapine/norclozapine ratio for schizophrenia management, and the principles governing use of this information will be distilled into 3 succinct axioms to aid clinicians in managing their clozapine-treated patients with schizophrenia.

Drug-drug interactions involving combinations of antipsychotic agents with antidiabetic, lipid-lowering, and weight loss drugs

INTRODUCTION

Patients with severe mental illness (SMI) have a high risk for diabetes, dyslipidemia, and other components of metabolic syndrome. Patients with these metabolic comorbidities and cardiac risk factors should receive not only antipsychotics but also medications aiming to reduce cardiovascular risk. Therefore, many patients may be exposed to clinically relevant drug-drug interactions.

AREAS COVERED

This narrative review summarizes data regarding the known or potential drug-drug interactions between antipsychotics and medications treating metabolic syndrome components, except for hypertension, which has been summarized elsewhere. A literature search in PubMed and Scopus up to 7/31/2021 was performed regarding interactions between antipsychotics and drugs used to treat metabolic syndrome components, aiming to inform clinicians' choice of medication for patients with SMI and cardiometabolic risk factors in need of pharmacologic interventions.

EXPERT OPINION

The cytochrome P450 system and, to a lesser extent, the P-glycoprotein transporter is involved in the pharmacokinetic interactions between antipsychotics and some statins or saxagliptin. Regarding pharmacodynamic interactions, the available information is based mostly on small studies, and for newer classes, like PCSK9 inhibitors or SGLT2 inhibitors, data are still lacking. However, there is sufficient information to guide clinicians in the process of selecting safer antipsychotic-cardiometabolic risk reduction drug combinations.

Using therapeutic drug monitoring to personalize clozapine dosing in Asians

This narrative review on clozapine blood levels or therapeutic drug monitoring (TDM) includes sections focused on drug clearance and TDM, personalized dosing with TDM, clinical applications of TDM in Asians, and areas needing further study. Asian patients need half the clozapine dose (D) used in the United States to get the same blood concentrations (C). The concentration-to-dose (C/D) ratio measures drug clearance. In the United States, the average clozapine patient usually needs from 300 to 600 mg/day to reach 350 ng/mL. US male smokers reach this therapeutic C with a D of 600 mg/day (C/D ratio of 0.60 = 600/350), whereas US female nonsmokers usually need a D of 300 mg/day (C/D ratio of 1.17 = 300/350). While in the United States, average CLO C/D ratios typically are 0.6-1.2 ng/mL per mg/day, in Asian populations they range from 1.20 in male smokers to 2.40 in female smokers, requiring Ds of 300 to 150 mg/day to obtain 350 ng/mL. Asian patients can become clozapine poor metabolizers (PMs), needing very low Ds (50-150 mg/day) to get therapeutic Cs, by taking inhibitors (fluvoxamine, oral contraceptives and valproic acid), due to obesity, or during inflammations with systemic effects. In 573 Asian patients from five samples, around 1% were PMs due to taking inhibitors, 1% due to inflammation, 1% due to obesity, and 7% were potential genetic PMs. The potential genetic PMs ranged between 3% and 13%, but this prevalence will have to be better established in future studies including genetic testing for possible CYP1A2 mutations, which may explain PM status.

A Rational Use of Clozapine Based on Adverse Drug Reactions, Pharmacokinetics, and Clinical Pharmacopsychology

Using Richardson and Davidson's model and the sciences of pharmacokinetics and clinical pharmacopsychology, this article reviewed the: (1) poor life expectancy associated with treatment-resistant schizophrenia (TRS), which may be improved in patients who adhere to clozapine; (2) findings that clozapine is the best treatment for TRS (according to efficacy, effectiveness and well-being); and (3) potential for clozapine to cause vulnerabilities, including potentially lethal adverse drug reactions such as agranulocytosis, pneumonia, and myocarditis. Rational use requires: (1) modification of the clozapine package insert worldwide to include lower doses for Asians and to avoid the lethality associated with pneumonia, (2) the use of clozapine levels for personalizing dosing, and (3) the use of slow and personalized titration. This may make clozapine as safe as possible and contribute to increased life expectancy and well-being. In the absence of data on COVID-19 in clozapine patients, clozapine possibly impairs immunological mechanisms and may increase pneumonia risk in infected patients. Psychiatrists should call their clozapine patients and families and explain to them that if the patient develops fever or flu-like symptoms, the psychiatrist should be called and should consider halving the clozapine dose. If the patient is hospitalized with pneumonia, the treating physician needs to assess for symptoms of clozapine intoxication since halving the dose may not be enough for all patients; consider decreasing it to one-third or even stopping it. Once the signs of inflammation and fever have disappeared, the clozapine dose can be slowly increased to the prior dosage level.

Personalizing dosing of risperidone, paliperidone and clozapine using therapeutic drug monitoring and pharmacogenetics

By combining knowledge of pharmacogenetics, therapeutic drug monitoring (TDM) and drug-drug interactions (DDIs) the author developed a model for personalizing antipsychotic dosing, which is applied to risperidone, 9-hydroxyrisperidone or paliperidone, and clozapine. Drugs are approved using an average dose for an ideal average patient, but pharmacologists have described outliers: genetic poor metabolizers (PMs) and ultrarapid metabolizers (UMs). Environmental and personal variables can also make patients behave as PMs or UMs. Drug clearance is represented by the concentration-to-dose (C/D) ratio under steady-state and trough conditions. A very low C/D ratio indicates a UM, while a very high C/D ratio indicates a PM. Total risperidone C/D ratio for the oral formulation is around 7 ng/ml per mg/day and can be influenced by CYP2D6 polymorphism, DDIs with inducers and inhibitors, and renal function. Oral paliperidone has low availability; its C/D ratio is around 4.1 ng/ml per mg/d and can be influenced by inducers and renal impairment. Once-a-month long-acting paliperidone provides a C/D ratio around 7.7 ng/ml per mg/day at steady state, which is expected to be in the 8th month (before the 9th injection). TDM is particularly important for long-acting paliperidone formulations that may accumulate once steady state is reached (after years for the 3- and 6-month formulations). In the US, clozapine C/D ratios typically range from 0.6 (male smokers) to 1.2 (female non-smokers) ng/ml per mg/day. East Asians' clozapine C/D ratios appear to be twice as high. Inhibitors (including fluvoxamine and oral contraceptives) and inflammation can also increase clozapine C/D ratios. This article is part of the issue entitled 'Special Issue on Antipsychotics'.