How Solvents Affect Acetaminophen Etching Pattern Formation: Interaction between Solvent and Acetaminophen at the Solid/Liquid Interface

Dissolution Profiles of Carbamazepine Cocrystals with Cis-Trans Isomeric Coformers

PURPOSE

The purpose of the present study was to investigate the dissolution profiles of cocrystals with cis-trans isomeric coformers. Previously, the carbamazepine (CBZ) cocrystals with even-carbon dicarboxylic acids showed higher supersaturation than those with odd-carbon ones, attributed to particle surface solution-mediated phase transformation (PS-SMPT) to CBZ dihydrate (CBZ DH). However, it has been unknown whether this odd-even pattern holds for cis-trans isomeric coformers.

METHOD

CBZ cocrystals with maleic acid (MLE) and fumaric acid (FUM) (CBZ-FUM anhydrate (CBZ-FUM AH) and monohydrate (CBZ-FUM H₂O)) were employed as model cocrystals. Hydroxypropyl　methylcellulose (HPMC), polyvinylpyrrolidone, and polyethylene glycol 6000 (PEG) were used as precipitation inhibitors. Dissolution tests were performed under a non-sink condition. Residual particles were analyzed by powder X-ray diffraction, differential scanning calorimetry, polarized light microscope, and scanning electron microscope.

RESULTS

All cocrystals showed little supersaturation in the absence of a polymer. In 0.1% HPMC, CBZ-FUM AH showed significant supersaturation, whereas CBZ-MLE and CBZ-FUM H₂O did not for the first two hours. HPMC reduced the initial dissolution rate of CBZ-MLE and CBZ-FUM H₂O while inducing the highest supersaturation among the polymers after 96 h. The particle surface changed from a smooth plane to a striped pattern, but little or no CBZ DH was detected.

CONCLUSION

The cocrystals with cis-trans isomeric coformers showed different dissolution profiles. HPMC increased the dissolution rate of CBZ-FUM AH by inhibiting PS-SMPT but reduced the dissolution rate of CBZ-MLE and CBZ-FUM H₂O without inducing PS-SMPT. The striped pattern was suggested to be due to surface etching rather than PS-SMPT.

Understanding the Effects of a Polymer on the Surface Dissolution of Pharmaceutical Cocrystals Using Combined Experimental and Molecular Dynamics Simulation Approaches

The molecular interactions between the surfaces of cocrystals [i.e., flufenamic acid and theophylline (FFA-TP), flufenamic acid and nicotinamide (FFA-NIC), and carbamazepine and nicotinamide (CBZ-NIC)] and the polymers [i.e., polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and copolymer of vinylpyrrolidone (60%)/vinyl acetate (40%) (PVP-VA)] were investigated through combined experimental and molecular dynamics simulation approaches to resolve the mechanisms of cocrystal dissolution and precipitation. It was found that adsorption of the polymers on the surfaces of cocrystals might prevent the precipitation of the parent drug and alter the dissolution rate. The effect of polymers on precipitation could be determined by the cocrystal dissolution rate, the interactions of polymers with the surfaces of cocrystals, the characters of the noncovalent bonds formed between the polymers and the cocrystal surfaces, and the mobility and conformation of the polymers. The etching experiments of single cocrystals revealed that FFA-NIC and CBZ-NIC appeared as surface precipitation cocrystals while FFA-TP could lead to bulk precipitation. Both PVP and PVP-VA were good precipitation inhibitors for FFA-NIC, and they could completely inhibit the recrystallization of FFA III on the surfaces of dissolving cocrystals. In addition, as the adsorption of the polymer was slower than dissolution rate of the cocrystals, PVP and PVP-VA could only partially inhibit the recrystallization of CBZ dihydrate on the surface of CBZ-NIC. While PEG had no inhibitory effect on the surface crystallization of FFA-NIC and CBZ-NIC, due to its weak interactions with the surfaces of the cocrystals, it enhanced the dissolution performance of FFA-TP. In contrast, PVP and PVP-VA reduced the dissolution rate of FFA-TP and subsequently undermined the performance of cocrystals. Taken together, the approach of combining experimental and molecular dynamics simulation provided insights into the mechanisms of cocrystal dissolution as well as the polymers acting as inhibitory excipients for precipitation/recrystallization, making contribution to the development of novel formulations.

Interaction Study of Paracetamol with Saturated (Capric) and Unsaturated (Oleic) Fatty Acids

Interaction study of paracetamol with saturated and unsaturated fatty acids, namely, capric and oleic acid have been performed by using serial dilution method, release behavior, FT-IR, and DSC study. Preliminary investigations by release studies indicated the possibility of interaction between paracetamol and fatty acids. UV-studies failed to detect any interaction between paracetamol and fatty acids. The possibility of hydrogen bonding between amino group of paracetamol and carbonyl group of fatty acids was revealed by FT-IR study. Polymorphic transition of paracetamol in the binary sample of paracetamol-capric acid was identified by DSC studies. However, no such possibility was detected in paracetamol-oleic acid mixture.

Study on the interactions between polyvinylpyrrolidone (PVP) and acetaminophen crystals: partial dissolution pattern change

The objective of the study was to investigate the interactions between polyvinylpyrrolidone (PVP) and acetaminophen crystal especially on crystal surface. The effects of PVP on the etching pattern change of the acetaminophen (010) face, solubility enhancement as well as the intrinsic dissolution rate (IDR) of acetaminophen crystals have been studied. The etching patterns of the acetaminophen (010) face in the presence of PVP have stable ledges in the direction of a-axis, but deviate from c-axis, which shows that the dissolution on the (010) face has been affected by the adsorbed PVP especially in the direction of c-axis through van der Waals interactions rather than hydrogen bonding interactions. Even though PVP(K30) can enhance the solubility of acetaminophen in concentration higher than 1 mg/mL, the IDR of acetaminophen in diluted PVP solutions was lower than in water. Because the viscosity of those diluted PVP(K30) solutions were the same as water viscosity, the lowered IDR of acetaminophen cannot be explained simply by the viscosity effects of PVP solutions. Overall, the study suggests that the PVP molecules adsorbed on the surface of acetaminophen crystals play an important role in etching pattern change as well as the intrinsic dissolution rate change.

Hydrogen bonding interactions between adsorbed polymer molecules and crystal surface of acetaminophen

The objective of this work was to investigate whether or not the hydrogen bonding interaction between polymer and crystal surface can be detected by the etching pattern changes in the presence of polymers. The (010) face of acetaminophen single crystal was used as a model solid surface. The etching patterns on the (010) face of acetaminophen crystal by water are in the directions of a- and c-axes, which are the same as the directions of the dominant attachment energies on the (010) face. In the presence of polymer, the hydrogen bonding interactions between adsorbed polymer and crystal surface can affect surface diffusion of acetaminophen molecules and change the etching patterns in the direction of a-axis, i.e., the direction of one hydrogen bond chain. Studies with 2-hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxypropyl methylcellulose (HPMC) and poly(vinyl alcohol) (PVA) showed that polymers, which can form hydrogen bonds with acetaminophen crystal surface, can change etching patterns in the direction of a-axis. Study with Dextran suggested that if a polymer cannot form hydrogen bonds with crystal surface due to steric repulsion, it will not change the etching pattern in the direction of a-axis. Studies with poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) further confirmed that only if a polymer can form hydrogen bonds with acetaminophen on crystal surface, the etching patterns in the direction of a-axis will be affected. The study results suggest that in the presence of polymers, the etching pattern change in the direction of hydrogen bond chain, the a-axis of acetaminophen crystals, can be used to indicate the existence of the hydrogen bonding interactions between adsorbed polymers and acetaminophen crystal surface.