Normal fat mass cannot be reliably estimated in typical pharmacokinetic studies

PURPOSE

An influential covariate for pharmacokinetics is (body) size. Recently, the method of estimation of normal fat mass (NFM) has been advocated. Here, the relative contribution of fat mass, estimated as a fraction fat (Ffat), is used to explain differences in pharmacokinetic parameters. This concept is more and more applied. However, it remains unclear whether NFM can be reliably estimated in these typical studies.

METHODS

We performed an evaluation of the reliability of NFM estimation in a typical study size (n = 30), otherwise best-case scenario, by means of a pharmacokinetic simulation study. Several values of Ffat were investigated.

RESULTS

In a typical pharmacokinetic study, high imprecision was observed for NFM parameter estimates over a range of scenarios. For example, in a scenario where the true value of Ffat on clearance was 0.5, we found a 95% confidence interval of - 0.1 to 2.1, demonstrating a low precision. The implications for practice are that one could conclude that fat-free mass best describes the relationship of the pharmacokinetics with body size, while the true relationship was between fat-free mass and total body weight. Consequently, this could lead to incorrect extrapolation of pharmacokinetics to extreme body sizes.

CONCLUSION

In typical pharmacokinetic studies, NFM should be used with caution because the Ffat estimates have low precision. The estimation of Ffat should always be preceded by careful study design evaluation before planning a study, to ensure that the design and sample size is sufficient to apply this potentially useful methodology.

Pharmacokinetics of HIV-Integrase Inhibitors During Pregnancy: Mechanisms, Clinical Implications and Knowledge Gaps

Prevention of mother-to-child transmission of HIV and optimal maternal treatment are the most important goals of antiretroviral therapy in pregnant women with HIV. These goals may be at risk due to possible reduced exposure during pregnancy caused by physiological changes. Limited information is available on the impact of these physiological changes. This is especially true for HIV-integrase inhibitors, a relatively new class of drugs, recommended first-line agents and hence used by a large proportion of HIV-infected patients. Therefore, the objective of this review is to provide a detailed overview of the pharmacokinetics of HIV-integrase inhibitors in pregnancy. Second, this review defines potential causes for the change in pharmacokinetics of HIV-integrase inhibitors during pregnancy. Despite increased clearance, for raltegravir 400 mg twice daily and dolutegravir 50 mg once daily, exposure during pregnancy seems adequate; however, for elvitegravir, the proposed minimal effective concentration is not reached during pregnancy. Lower exposure to these drugs may be caused by increased hormone levels and, subsequently, enhanced drug metabolism during pregnancy. The pharmacokinetics of bictegravir and cabotegravir, which are under development, have not yet been evaluated in pregnant women. New studies need to prospectively assess whether adequate exposure is reached in pregnant women using these new HIV-integrase inhibitors. To further optimize antiretroviral treatment in pregnant women, studies need to unravel the underlying mechanisms behind the changes in the pharmacokinetics of HIV-integrase inhibitors during pregnancy. More knowledge on altered pharmacokinetics during pregnancy and the underlying mechanisms contribute to the development of effective and safe antiretroviral therapy for HIV-infected pregnant women.

Pharmacokinetics of HIV-Integrase Inhibitors During Pregnancy: Mechanisms, Clinical Implications and Knowledge Gaps

Prevention of mother-to-child transmission of HIV and optimal maternal treatment are the most important goals of antiretroviral therapy in pregnant women with HIV. These goals may be at risk due to possible reduced exposure during pregnancy caused by physiological changes. Limited information is available on the impact of these physiological changes. This is especially true for HIV-integrase inhibitors, a relatively new class of drugs, recommended first-line agents and hence used by a large proportion of HIV-infected patients. Therefore, the objective of this review is to provide a detailed overview of the pharmacokinetics of HIV-integrase inhibitors in pregnancy. Second, this review defines potential causes for the change in pharmacokinetics of HIV-integrase inhibitors during pregnancy. Despite increased clearance, for raltegravir 400 mg twice daily and dolutegravir 50 mg once daily, exposure during pregnancy seems adequate; however, for elvitegravir, the proposed minimal effective concentration is not reached during pregnancy. Lower exposure to these drugs may be caused by increased hormone levels and, subsequently, enhanced drug metabolism during pregnancy. The pharmacokinetics of bictegravir and cabotegravir, which are under development, have not yet been evaluated in pregnant women. New studies need to prospectively assess whether adequate exposure is reached in pregnant women using these new HIV-integrase inhibitors. To further optimize antiretroviral treatment in pregnant women, studies need to unravel the underlying mechanisms behind the changes in the pharmacokinetics of HIV-integrase inhibitors during pregnancy. More knowledge on altered pharmacokinetics during pregnancy and the underlying mechanisms contribute to the development of effective and safe antiretroviral therapy for HIV-infected pregnant women.

Association of concomitant use of acid reducing agents in full-dose vemurafenib users with risk of progression in BRAF V600 mutation-positive unresectable or metastatic melanoma patients: A retrospective cohort study

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Background: Vemurafenib is used for the treatment of adult patients with BRAF V600 mutation-positive unresectable or metastatic melanoma. The approved fixed vemurafenib dose of 960 mg twice daily may result in overexposure. Concomitant use of acid reducing agents (ARAs) may result in underexposure. Both situations are likely to affect treatment outcome. Therefore, the aim of this study was to determine the association between the use of vemurafenib (full-dose versus reduced dose) and/or concomitant ARA use (yes versus no) and the risk of disease progression. Methods: A retrospective cohort study was conducted using data from the electronic health record software of the Radboudumc pharmacy and medical records of the Radboudumc (March 17th 2012 to March 17th 2016). Patients (N = 112) using vemurafenib as first line treatment for melanoma were included. Multivariable cox regression estimated adjusted hazard ratios (HRa) and 95% confidence intervals (CI) of progression in vemurafenib users (full-dose N = 67 versus reduced dose N = 45) and/or concomitant ARA users (N = 38). Adjustments were made for age and sex. Results: The mean follow-up time was 3.5 months and 41 patients (36.6%) developed progression on first line vemurafenib. Co-treatment of ARAs in patients using full-dose vemurafenib was associated with a 4.6-fold increased risk of progression (HRa 4.56; 95% CI 1.51-13.75) as compared to full-dose vemurafenib users not co-treated with ARAs. No increased risk was found for users of vemurafenib in a reduced dose, regardless of concomitant ARA use. Conclusions: Concomitant use of ARAs in full-dose vemurafenib users was associated with an increased risk of progression. Physicians should be cautious to prescribe ARAs to patients tolerating full-dose vemurafenib. The presence of considerable confounding by disease severity, the small number of events and the hypothesis generating character of this study emphasize the need to prospective validate these results.