Background: Sunitinib is an inhibitor of multiple receptor tyrosine kinases and is a standard-of-care treatment for advanced and metastatic renal cell carcinoma and a second-line treatment in locally advanced inoperable and metastatic gastrointestinal stromal tumors. A fixed dose of the drug, however, does not produce a uniform therapeutic outcome in all patients, and many face adverse effects and/or toxicity. One of the possible causes of the interindividual variability in the efficacy and toxicity response is the highly variable systemic exposure to sunitinib and its active metabolite. This review aims to summarize all available clinical evidence of the treatment of adult patients using sunitinib in approved indications, addressing the necessity to introduce proper and robust therapeutic drug monitoring (TDM) of sunitinib and its major metabolite, N-desethylsunitinib. Methods: The authors performed a systematic search of the available scientific literature using the PubMed online database. The search terms were “sunitinib” AND “therapeutic drug monitoring” OR “TDM” OR “plasma levels” OR “concentration” OR “exposure.” The search yielded 520 journal articles. In total, 447 publications were excluded because they lacked sufficient relevance to the reviewed topic. The remaining 73 articles were, together with currently valid guidelines, thoroughly reviewed. Results: There is sufficient evidence confirming the concentration–efficacy and concentration–toxicity relationship in the indications of gastrointestinal stromal tumors and metastatic renal clear-cell carcinoma. For optimal therapeutic response, total (sunitinib + N-desethylsunitinib) trough levels of 50–100 ng/mL serve as a reasonable target therapeutic range. To avoid toxicity, the total trough levels should not exceed 100 ng/mL. Conclusions: According to the current evidence presented in this review, a TDM-guided dose modification of sunitinib in selected groups of patients could provide a better treatment outcome while simultaneously preventing sunitinib toxicity.

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Objective:Currently available methods for endogenous cortisol monitoring in patients with hormonal insufficiency rely on measurements of plasma levels only at a single time point; thus, any kind of chronic exposure to cortisol is challenging to evaluate because it requires collecting samples at different time points. Hair cortisol levels acquired longitudinally better reflected chronic exposure (both cortisol synthesis and deposition) and may significantly contribute to better outcomes in glucocorticoid replacement therapies.Design:Twenty-two patients on cortisol substitution therapy were monitored for plasma, urinary, and hair cortisol levels for 18 months to determine whether hair cortisol may serve as a monitoring option for therapy setting and adjustment.Methods:Plasma and urinary cortisol levels were measured using standardized immunoassay methods, and segmented (∼1 cm) hair cortisol levels were monitored by liquid chromatography coupled to mass spectrometry. A log-normal model of the changes over time was proposed, and Bayesian statistics were used to compare plasma, urinary, and hair cortisol levels over 18 months.Results and conclusions:Hair cortisol levels decreased over time in patients undergoing substitutional therapy. The residual variance of hair cortisol in comparison to plasma or urinary cortisol levels was much lower. Thus, longitudinal monitoring of hair cortisol levels could prove beneficial as a noninvasive tool to reduce the risk of overdosing and improve the overall patient health.

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