Elucidation of inclusion compounds between b-cyclodextrin/local anaesthetics structure: a theoretical and experimental study using differential scanning calorimetry and molecular mechanics Luciana M.A. Pinto a , Marcelo B. de Jesus a , Eneida de Paula a , Antonio C.S. Lino b, * , Joel B. Alderete c , Helio A. Duarte d , Yuji Takahata b a Departamento de Bioquı ´mica, Instituto de Biologia, Universidade Estadual de Campinas, Cidade Universita ´ria Zeferino Vaz, C.P. 6109, CEP: 13 083-970, Campinas, SP, Brazil b Departamento de Fı ´sico-Quı ´mica, Instituto de Quı ´mica, Universidade Estadual de Campinas, Cidade Universita ´ria Zeferino Vaz, C.P. 6154, CEP: 13083-970, Campinas, SP, Brazil c Organic Chemistry Department, Universidad Concepcion, Casilla 3C, Concepcion, Chile d Departamento de Quı ´mica, ICEx, Universidade Federal de Minas Gerais, Av. Antonio Carlos no 6627, Pampulha, CEP: 31270-910, Belo Horizonte, MG, Brazil Received 1 December 2003; accepted 9 February 2004 Abstract Two local anaesthetics, Bupivacaine (BVC) and Lidocaine (LDC), were studied by experimental as well as theoretical methods. These compounds were associated with a drug delivery system of cyclodextrins (CDs). We used Differential Scanning Calorimetry (DSC) for experimental measurements. We calculated enthalpy change for the process of association between drug and CD. Calculations were performed using Molecular Mechanics (MM) with MM þ and OPLS Forced Fields. Both 1:1 and 1:2 stoichiometries were studied. DSC result indicates formation of stable inclusion compounds for all cases. A difference between physical mixture and inclusion compound was observed. The OPLS GB/SA (OPLSw) results show that the difference of enthalpy change between 1:1 and 1:2 stoichiometries is ca. ^ 1 kcal mol 21 for BVC. For LDC, the corresponding difference is ca. 10 kcal mol 21 . This indicates that ring out type of docking is stable for the 1:1 stoichiometry. Head-to-head alignment was predicted to be stable in 1:2 stoichiometries. 1:1 and 1:2 associations coexist and establish a dynamic equilibrium. q 2004 Elsevier B.V. All rights reserved. Keywords: Cyclodextrins; Local anaesthetics; Inclusion compounds; Molecular mechanics; Differential scanning calorimeter 1. Introduction Local anaesthetics (LA) are a class of medicines widely used to provide pain relief. They are amphiphilic molecules that act along the axon inhibiting the Na þ influx and thus the signal transduction. Bupivacaine (BVC) and Lidocaine (LDC) are two of the most widely used LA in clinics. BVC belongs to the cyclic amino-amide class and is one of the most potent LA used nowadays in epidural anaesthesia and it is very hydrophobic. On the other hand, LDC belongs to the non-cyclic amine-amide class, has hydrophilic features and has been applied to infiltrative anaesthesia, as well as topical ones. In this work, the possibility of manufacturing medicines with BVC and LDC by using b-cyclodextrin (b-CD) as the drug delivery system is demonstrated. It could lead to formulations with different physico-chemical properties, increasing the potency and bioavailability of such molecules. b-CD is a polysaccharide derived from enzymatic degradation of starch using Baccilus Amylobac- ter. It is formed by the connection of seven glycosidic units by a, 1 – 4, C – O – C linkage giving rise to a geometry like a cup. The molecular geometry of CD is considered a perfect geometry for inclusion compounds. The toroidal shape can fit substituted benzene ring in a 1:1 or 1:2 or other drug:CD stoichiometry [1,2]. A recent work showed the possibility to elucidate the differences on the stability of 1:1 or 1:2 stoichiometries based upon a simultaneous experimental 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.02.036 Journal of Molecular Structure (Theochem) 678 (2004) 63–66 www.elsevier.com/locate/theochem * Corresponding author. E-mail address: acslino@hotmail.com (A.C.S. Lino).