Application of Selective Crystallization Methods To Isolate the Metastable Polymorphs of Paracetamol: A Review

Polymorphism and Multi-Component Crystal Formation of GABA and Gabapentin

This study exploits the polymorphism and multi-component crystal formation of γ-amino butanoic acid (GABA) and its pharmaceutically active derivative, gabapentin. Two polymorphs of GABA and both polymorphs of gabapentin are structurally revisited, together with gabapentin monohydrate. Hereby, GABA form II is only accessible under special conditions using additives, whereas gabapentin converts to the monohydrate even in the presence of trace amounts of water. Different accessibilities and phase stabilities of these phases are still not fully clarified. Thus, indicators of phase stability are discussed involving intermolecular interactions, molecular conformations, and crystallization environment. Calculated lattice energy differences for polymorphs reveal their similar stability. Quantification of the hydrogen bond strengths with the atoms-in-molecules (AIM) model in conjunction with non-covalent interaction (NCI) plots also shows similar hydrogen bond binding energy values for all polymorphs. We demonstrate that differences in the interacting modes, in an interplay with the intermolecular repulsion, allow the formation of the desired phase under different crystallization environments. Salts and co-crystals of GABA and gabapentin with fumaric as well as succinic acid further serve as models to highlight how strongly HBs act as the motif-directing force in the solid-phase GABA-analogs. Six novel multi-component entities were synthesized, and structural and computational analysis was performed: GABA fumarate (2:1); two gabapentin fumarates (2:1) and (1:1); two GABA succinates (2:1) and (1:1); and a gabapentin:succinic acid co-crystal. Energetically highly attractive carboxyl/carboxylate interaction overcomes other factors and dominates the multi-component phase formation. Decisive commonalities in the crystallization behavior of zwitterionic GABA-derivatives are discussed, which show how they can and should be understood as a whole for possible related future products.

Crystallization of Form II Paracetamol with the Assistance of Carboxylic Acids toward Batch and Continuous Processes

Form II paracetamol has captured the interest of researchers due to its improved compressibility. However, its low stability has made it difficult to be produced on a large scale with good reproducibility. In the present study, the selective polymorphic formation of paracetamol was carried out by cooling crystallization with four types of additives: adipic acid, fumaric acid, oxalic acid, and succinic acid. It was found that: (1) the more additives that were added, the higher the probability of forming Form II paracetamol; (2) Form II paracetamol could be induced by seeding the paracetamol aqueous solution with Form II paracetamol and fumaric acid crystals, and not the other three carboxylic acids; (3) a new solution complex of paracetamol-oxalic acid, evidenced by the solubility diagram, was responsible for the selective nucleation of Form II paracetamol in the oxalic acid aqueous solution; and (4) the range of the degree of supersaturation for nucleating Form II paracetamol was extended with the assistance of oxalic acid or fumaric acid. In large-scale crystallization, Form II paracetamol was produced by the continuous crystallization of 44 mg of paracetamol/mL in 50 wt% of fumaric acid aqueous solution with a flow rate of 150 mL/min.

Selective formation of form II paracetamol through the assistance of paracetamol co-crystals as templates in a solution

The arrangement of paracetamol templated by the structure of 1 : 1 co-crystal of paracetamol–maleic acid in solution phase facilitating the nucleation of metastable form II paracetamol.