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Abstract Introduction: Mitotane (o,p’–DD) is the drug of choice for Adrenocortical Carcinomas (ACC) and its measurement in plasma is essential to control drug administration. Objective: To develop and validate a simple, reliable and straightforward method for mitotane determination in plasma samples. Method: Drug-free plasma samples were collected in potassium-ethylenediamine tetraacetate (K-EDTA) tubes and spiked with 1.0, 2.5, 10.0, 25.0 and 50.0 µg/mL of mitotane (DDD). The p,p’-DDD was used as an Internal Standard (IS) and was added at 25.0 µg/mL concentration to all samples, standards and controls. Samples were submitted to protein precipitation with acetonitrile and then centrifuged. 50 uL of the supernatant was injected into an HPLC system coupled to a Diode Array Detector (DAD). DDD and IS were detected at 230 nm in a 12 min isocratic mode with a solvent mixture of 60 % acetonitrile and 40 % formic acid in water with 0.1 % pump mixed, at 0.6 mL/min flow rate, in a reversed-phase (C18) chromatographic column kept at 28°C. The sensitivity, selectivity, precision, presence of carry-over, recovery and matrix-effect, linearity, and method accuracy were evaluated. Results: The present study’s method resulted in a symmetrical peak shape and good baseline resolution for DDD (mitotane) and 4,4’-DDD (internal standard) with retention times of 6.0 min, 6.4 mim, respectively, with resolutions higher than 1.0. Endogenous plasma compounds did not interfere with the evaluated peaks when blank plasma and spiked plasma with standards were compared. Linearity was assessed over the range of 1.00 -50.00 µg/mL for mitotane (R2 > 0.9987 and a 97.80 %-105.50 % of extraction efficiency). Analytical sensitivity was 0.98 µg/mL. Functional sensitivity (LOQ) was 1.00 µg/L, intra-assay and inter-assay coefficient of variations were less than 9.98 %, and carry-over was not observed for this method. Recovery ranged from 98.00 % to 117.00 %, linearity ranged from 95.00 % to 119.00 %, and high accuracy of 89.40 % to 105.90 % with no matrix effects or interference was observed for mitotane measurements. Patients’ sample results were compared with previous measurements by the GC-MS method with a high correlation (r = 0.88 and bias = −10.20 %). Conclusion: DDD determination in plasma samples by the developed and validated method is simple, robust, efficient, and sensitive for therapeutic drug monitoring and dose management to achieve a therapeutic index of mitotane in patients with adrenocortical cancer. Introduction o,p–DD opDD o,p –DD o p DD (o,p’–DD ACC (ACC administration Objective simple Method Drugfree Drug free potassiumethylenediamine potassium ethylenediamine KEDTA K EDTA (K-EDTA 10 1 0 1.0 25 2 5 2.5 100 10.0 250 25. 500 50. µgmL µg mL DDD. . (DDD) p,pDDD ppDDD p,p (IS controls centrifuged DAD. DAD (DAD) 23 6 4 01 0. mixed 06 mLmin rate reversedphase reversed phase C18 C (C18 28C 28 28°C selectivity precision carryover, carryover carry over, matrixeffect, matrixeffect effect, effect matrix-effect Results studys study s (mitotane 4,4DDD 44DDD 4,4 internal standard 6. 64 mim respectively 00 50.00 5000 -50.0 R2 R (R 09987 9987 0.998 9780 97 80 97.8 %105.50 10550 105.50 105 %-105.5 efficiency. efficiency efficiency) 098 98 0.9 LOQ (LOQ µgL L µg/L intraassay intra assay interassay inter 998 9 9.9 9800 98.0 11700 117 117.0 9500 95 95.0 11900 119 119.0 8940 89 89.4 10590 90 105.9 Patients GCMS GC MS r 088 88 0.8 1020 20 −10.2 %. %) Conclusion robust efficient cancer op 1. 2. 10. (DDD pDDD pp (DAD C1 (C1 4DDD 44 4, -50. 0998 0.99 978 8 97. %105.5 1055 105.5 %-105. 09 99 9. 980 98. 1170 11 117. 950 95. 1190 119. 894 89. 1059 105. 08 102 −10. (C -50 099 %105. %-105 −10 -5 %105 %-10 −1 - %10 %-1 − %1 %-