Biophysical studies of human serum albumin with cocaine and cocaethylene: Understanding the drug-protein interaction in simulated physiological conditions

In this study, HSA was used as a transport protein model to simulate the transport dynamics and availability of cocaine (COC) and its pharmacologically active metabolite, cocaethylene (CE). In the interaction studies between the alkaloids and HSA, the binding constant (K_b) values were higher for CE (2.92 to 8.33 × 10⁴ M⁻¹) than COC (1.15 to 3.77 × 10⁴ M⁻¹) at different temperatures. Thermodynamic parameter calculations indicate that COC preferentially interacts through van der Waals forces and hydrogen bonds, while CE interacts hydrophobically. Competition studies using the ANS probe confirmed the more hydrophobic characteristics of CE. Synchronized fluorescence indicated that both compounds preferentially interact in the microregion of the Trp214 residue. Based on 3D fluorescence, UV–vis, and circular dichroic results, changes in the secondary structure of the protein were confirmed. Studies to assess the HSA binding site were carried out using warfarin (site I), diazepam (site II), and digitoxin (subdomain IIIB) as markers, and it was verified that both compounds interact preferentially at the site I. Finally, based on the ¹H NMR and theoretical studies, it was possible to propose the alkaloid epitope in the interaction with HSA. In conclusion, when compared to COC, CE presented greater HSA protein affinity, justifying its longer plasma lifetime and increasing the drug’s effects on the body.