Sequential Molecular Events in the Solid-State Acid-Base Incompatibility of Amlodipine Besylate and Citric Acid: 2D-COS-PXRD and 2D-COS-FTIR as Orthogonal Probes of Reaction Sequence in Drug-Excipient Solid-State Incompatibility.

Drug-excipient incompatibility in solid oral dosage forms can generate toxic degradants and compromise potency without producing a visible macroscopic signal in early development. We reported, for the first time, a mechanistically resolved, temperature-sequenced characterization of the solid-state acid-base incompatibility between amlodipine besylate (AMLO; pKa ≈ 9.4) and citric acid (CA; pKa1 3.13; ΔpKa = 6.27) using six orthogonal techniques: DSC, TGA, isothermal titration calorimetry (ITC), powder X-ray diffractometry with two-dimensional correlation spectroscopy (2D-COS-PXR), FTIR with 2D-COS (2D-COS-FTIR), and stability-indicating HPLC coupled to LC-QToF HRMS. Binary mixtures were thermally stressed at 40-90 °C (7 day isothermal, closed-vial). ITC confirmed a spontaneous, entropy-dominated interaction (ΔG = -5.14 kcal/mol; -TΔS = -3.78 kcal/mol; N ≈ 1:1; KD = 197 μM) driven by proton transfer to the AMLO dihydropyridine nitrogen with concurrent besylate displacement. DSC showed complete loss of the AMLO Form-I endotherm (onset 201.01 °C), replaced by a single event at 155.57 °C with super-additive enthalpy (+19.50 J/g), consistent with amlodipine citrate salt formation. Application of Noda's 2D-COS algorithm to temperature-resolved PXRD and FTIR datasets independently resolved a four-stage mechanism, viz., CA surface proton mobilization (40 °C) → amlodipine citrate nucleation (50-60 °C) → besylate counter-ion displacement and d-spacing contraction (∼70 °C) and → partial amorphization and acid-catalyzed covalent degradation (80-90 °C). Three sequential-order reversals in the 2D-COS-FTIR asynchronous map independently corroborated the Stage-3 crystallographic sequence. HPLC revealed a 4.48-fold enhancement of AMLO loss in the mixture (7.97%) versus pure drug (1.78%), with LC-QToF HRMS identifying four mixture-unique degradants consistent with the four-stage model. This is also the first pharmaceutical application of 2D-COS-PXRD as a mechanistic probe of sequential crystallographic events in drug-excipient incompatibility.