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Abstract
A stability-indicating reversed-phase HPLC (RP-HPLC) method was developed and validated for the simultaneous quantification of palbociclib and diclofenac, with application to forced degradation and degradation-kinetic evaluation. Separation was achieved on a C18 column (250 × 4.6 mm, 5 µm) using acetonitrile–0.02 M potassium dihydrogen phosphate buffer (55:45, v/v; pH 3.2 ± 0.1) at 1.0 mL/min with UV detection at 265 nm and a 20 µL injection volume. Diclofenac and palbociclib were eluted at 3.42 and 6.80 min, respectively, with a resolution of 4.3. The method was linear over 2–40 µg/mL (palbociclib) and 1–20 µg/mL (diclofenac) with r² = 0.9999 for both analytes. Mean recoveries ranged from 99.10–100.12% (palbociclib) and 99.38–100.50% (diclofenac), and intra-/inter-day precision was within %RSD 0.45–0.71. LOD/LOQ values were 0.4/1.2 µg/mL for palbociclib and 0.2/0.7 µg/mL for diclofenac. Forced degradation under acidic, alkaline, oxidative, neutral hydrolysis, thermal, and photolytic conditions produced 2.5–22.0% degradation with mass balance of 99.3–100.0%, confirming specificity. Kinetic assessment indicated that ln(% remaining) versus time gave the best fit, consistent with first-order degradation under the selected stress conditions. Overall, the method is suitable for routine assay, stability testing, and kinetic profiling of palbociclib–diclofenac systems.