No inhibitory effect on P-glycoprotein function at blood–brain barrier by clinical dose of clarithromycin: a human PET study with [11C]verapamil

Safety and Tolerability of Antimicrobial Agents in the Older Patient

Older patients are at high risk of infections, which often present atypically and are associated with high morbidity and mortality. Antimicrobial treatment in older individuals with infectious diseases represents a clinical challenge, causing an increasing burden on worldwide healthcare systems; immunosenescence and the coexistence of multiple comorbidities determine complex polypharmacy regimens with an increase in drug-drug interactions and spread of multidrug-resistance infections. Aging-induced pharmacokinetic and pharmacodynamic changes can additionally increase the risk of inappropriate drug dosing, with underexposure that is associated with antimicrobial resistance and overexposure that may lead to adverse effects and poor adherence because of low tolerability. These issues need to be considered when starting antimicrobial prescriptions. National and international efforts have been made towards the implementation of antimicrobial stewardship (AMS) interventions to help clinicians improve the appropriateness and safety of antimicrobial prescriptions in both acute and long-term care settings. AMS programs were shown to decrease consumption of antimicrobials and to improve safety in hospitalized patients and older nursing home residents. With the abundance of antimicrobial prescriptions and the recent emergence of multidrug resistant pathogens, an in-depth review of antimicrobial prescriptions in geriatric clinical practice is needed. This review will discuss the special considerations for older individuals needing antimicrobials, including risk factors that shape risk profiles in geriatric populations as well as an evidence-based description of antimicrobial-induced adverse events in this patient population. It will highlight agents of concern for this age group and discuss interventions to mitigate the effects of inappropriate antimicrobial prescribing.

Use of PET Imaging to Evaluate Transporter-Mediated Drug-Drug Interactions

Several membrane transporters belonging to the adenosine triphosphate-binding cassette (ABC) and solute carrier (SLC) families can transport drugs and drug metabolites and thereby exert an effect on drug absorption, distribution, and excretion, which may potentially lead to transporter-mediated drug-drug interactions (DDIs). Some transporter-mediated DDIs may lead to changes in organ distribution of drugs (eg, brain, liver, kidneys) without affecting plasma concentrations. Positron emission tomography (PET) is a noninvasive imaging method that allows studying of the distribution of radiolabeled drugs to different organs and tissues and is therefore the method of choice to quantitatively assess transporter-mediated DDIs on a tissue level. There are 2 approaches to how PET can be used in transporter-mediated DDI studies. When the drug of interest is a potential perpetrator of DDIs, it may be administered in unlabeled form to assess its influence on tissue distribution of a generic transporter-specific PET tracer (probe substrate). When the drug of interest is a potential victim of DDIs, it may be radiolabeled with carbon-11 or fluorine-18 and used in combination with a prototypical transporter inhibitor (eg, rifampicin). PET has already been used both in preclinical species and in humans to assess the effects of transporter-mediated DDIs on drug disposition in different organ systems, such as brain, liver, and kidneys, for which examples are given in the present review article. Given the growing importance of membrane transporters with respect to drug safety and efficacy, PET is expected to play an increasingly important role in future drug development.

Retrospective analysis of P-glycoprotein-mediated drug-drug interactions at the blood-brain barrier in humans

To date, the in vitro-in vivo correlation (IVIVC) of P-glycoprotein (P-gp)-mediated drug-drug interaction (DDI) at the blood-brain barrier (BBB) in rats indicated that the cutoff value to significantly affect the brain penetration of digoxin was [I,unbound/Ki] of 1, where I,unbound is the unbound plasma concentration of P-gp inhibitors. On the basis of the IVIVC in rats, we speculated that clinically used P-gp inhibitors do not cause DDI at the human BBB, because none of the compounds studied was [I,unbound/Ki]>1 at therapeutic doses. Recently, positron emission tomography studies with P-gp substrates, such as [(11)C]verapamil, [(11)C]N-desmethyl loperamide, and [(11)C]loperamide, together with potent P-gp inhibitors, have indicated that increases in the influx rate constant for brain entry were observed in humans. Therefore, we aimed to retrospectively analyze the results of P-gp-mediated DDIs with in vitro P-gp inhibition assays and to confirm the appropriate cutoff value. In vitro P-gp inhibition assays using verapamil, N-desmethyl loperamide, and loperamide as P-gp probe substrates were performed in human multidrug resistance protein 1-expressing LLC-PK1 cells. The efflux ratios decreased in the presence of P-gp inhibitors, and the Ki of tariquidar was 10 nmol/L, regardless of probe substrates. Taking the in vitro Ki and unbound plasma concentrations in clinical DDI studies together, the criterion [I,unbound/Ki] of 1 was an appropriate cutoff limit to observe significant P-gp-mediated DDI at the BBB in humans. On the other hand, no significant DDI was observed in cases in which [I,unbound/Ki] was less than 0.1. This criterion was comparable to the previous IVIVC result in rats.