pubs.acs.org/crystal Published on Web 05/28/2010 r 2010 American Chemical Society DOI: 10.1021/cg100212q 2010, Vol. 10 3094–3101 Triple Structural Transition below Room Temperature in the Antifilarial Drug Diethylcarbamazine Citrate Cecilia C. P. da Silva, † Felipe T. Martins, † Sara B. Honorato, ‡ N ubia Boechat, § Alejandro P. Ayala, ‡ and Javier Ellena* ,† † Instituto de Fı´sica de S~ ao Carlos, Universidade de S~ ao Paulo, CP 369, S~ ao Carlos, S~ ao Paulo 13560-970, Brazil, ‡ Departamento de Fı´sica, Universidade Federal do Cear a, CP 6030, Fortaleza, Cear a 60455-970, Brazil, and § Fundac - ~ ao Oswaldo Cruz - FioCruz, Instituto de Tecnologia em F armacos-FarManguinhos, Rua Sizenando Nabuco 100, Rio de Janeiro, Rio de Janeiro 21041-250, Brazil Received February 10, 2010; Revised Manuscript Received April 20, 2010 ABSTRACT: A very unusual triple structural transition pattern below room temperature was observed for the antifilarial drug diethylcarbamazine citrate. Besides the first thermal, crystallographic, and vibrational investigations of this first-line drug used in clinical treatment for lymphatic filariasis, a noteworthy behavior with three structural transformations as a function of temperature was demonstrated by differential scanning calorimetry, Raman spectroscopy, and single-crystal X-ray diffractometry. Our X-ray data on single crystals allow for a complete featuring and understanding of all transitions, since the four structures associated with the three solid-solid phase transformations were accurately determined. Two of three structural transitions show an order-disorder mechanism and temperature hysteresis with exothermic peaks at 224 K (T 1 0 ) and 213 K (T 2 0 ) upon cooling and endothermic ones at 248 K (T 1 ) and 226 K (T 2 ) upon heating. The other transition occurs at 108 K (T 3 ) and it is temperature-rate sensitive. Molecular displacements onto the (010) plane and conformational changes of the diethylcarbamazine backbone as a consequence of the C-H 333 N hydrogen bonding formation/cleavage between drug molecules explain the mechanism of the transitions at T 1 0 /T 2 . However, such changes are observed only on alternate columns of the drug intercalated by citrate chains, which leads to a doubling of the lattice period along the a axis of the 235 K structure with respect to the 150 and 293 K structures. At T 2 0 /T 1 , these structural alterations occur in all columns of the drug. At T 3 , there is a rotation on the axis of the N-C bond between the carbamoyl moiety and an ethyl group of one crystallographically independent diethylcarbamazine molecule besides molecular shifts and other conforma- tional alterations. The impact of this study is based on the fascinating finding in which the versatile capability of structural adaptation dependent on the thermal history was observed for a relatively simple organic salt, diethylcarbamazine citrate. 1. Introduction Filariasis is an endemic disease in 83 tropical countries of Asia, Africa, and Central and South Americas. 1-5 About 120 million persons are infected worldwide. 6 Among other disease manifestations, chronic stages of lymphatic filariasis (LF) often present with lymphoedema, known also as elephantiasis. This worst symptom causes serious psychological and social damage to infected persons. 7,8 Diethylcarbamazine (1-(N,N-diethylcar- bamoyl)-4-methylpiperazine, DEC) is the first-line antifilarial drug for LF treatment. It acts by killing the parasitic filarial worms (mainly Wuchereria bancrofti) lodged in the lymphatic system. 9,10 DEC, in particular its citrate salt (DEC citrate) due to its most favorable solubility and stability compared to the free base and other salts, has been marketed for more than 50 years by several pharmaceutical companies under different trade names. 11-14 Even though this active pharmaceutical ingredient (API) has been known and incorporated into drug formulations for a long time, its solid state physical behavior has not been investigated. Therefore, vibrational, crystallographic, and thermal studies of DEC citrate were our first goals in an attempt to pioneer the characterization and understanding of their solid state proper- ties related to drug performance. Structure determination was the first step in featuring DEC citrate. In addition to the first solid state structure determination of this antifilarial drug reported here, we found three reversible structural transitions below room temperature that exhibit a very complex thermo- dynamics. Such triple structural transition behavior below room temperature is very rare. 15-18 Even more surprising than dis- covering three low temperature structural transformations and determining the four related crystal phases of DEC citrate was the fact that this triple phenomenon is still more unusual in the case of molecular solids composed only by organic frame- works, 19,20 mainly for molecular salts. Therefore, DEC citrate is an attractive case for solid state dynamics comprehension of such compounds. 2. Experimental Section 2.1. Crystal Growth. Transparent well-grown block-shaped DEC citrate crystal suitable for single-crystal X-ray diffraction analysis was selected after the crystallization procedure. To prepare such a crystal, raw material of DEC citrate (100 mg) from Fundac -~ ao Oswaldo Cruz (FioCruz, Instituto de Tecnologia em F armacos-FarManguinhos) was dissolved in ethanol (50 mL) by vigorous shaking of the mixture at 60 °C. Next, the mixture was filtered through a 0.25 μm filter (Millipore) at room temperature and the resulting solution was allowed to evaporate slowly for 15 days at 5 °C. 2.2. Single-Crystal X-ray Structure Determination. DEC citrate crystal structures of phase I, phase II, phase III, and phase IV were determined at 293, 235, 150, and 100 K, respectively. However, it is im- portant to note that exothermic peaks in differential scanning calori- metry (DSC) traces (see below) associated with the three structural transitions were observed at 224 K (temperature T 1 0 , from phase I to phase II) and 213 K (temperature T 2 0 , from phase II to phase III) upon cooling DEC citrate crystals, while upon heating the endothermic ones were observed at 248 K (temperature T 1 , from phase II to phase I), 226 K (temperature T 2 , from phase III to phase II), and 108 K *To whom correspondence should be addressed. E-mail: javiere@ ifsc.usp.br.