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Article pubs.acs.org/crystal
Co-crystal of Tramadol Hydrochloride−Celecoxib (ctc): A Novel API− API Co-crystal for the Treatment of Pain Carmen Almansa,*,† Ramon Mercè,† Nicolas Tesson,‡ Joan Farran,‡ Jaume Tomàs,† and Carlos R. Plata-Salamán† †
Laboratorios del Dr. Esteve, S.A.U., Barcelona 08028, Spain Enantia, S.L., Barcelona 08028, Spain
‡
ABSTRACT: We have identified a co-crystal of tramadol hydrochloride−celecoxib (ctc; E-58425/MR308), a novel active pharmaceutical ingredient (API)−API co-crystal formed by an intrinsic 1:1 molecular ratio of rac-tramadol·HCl and celecoxib, which displays favorable physicochemical and dissolution profiles. Adequate treatment of pain represents an unmet medical need that is often addressed via combination therapy. API−API co-crystals represent a new approach with potential to improve physicochemical properties, bioavailability, stability, or formulation capacity, which may translate into optimized pharmacokinetic profiles and clinical benefits compared with individual APIs or their combination. ctc showed a well-defined differential scanning calorimetry profile, and its single-crystal X-ray diffraction structure demonstrated a supramolecular 3D network in which the two active enantiomers of tramadol and celecoxib are linked via hydrogen bonding and chloride ions. Oversaturation studies indicated that the saturation effect for highly insoluble celecoxib occurred at a higher concentration in ctc than in celecoxib alone. Comparative intrinsic dissolution rate studies showed that the release of celecoxib was faster, and the release of tramadol was slower, from ctc than from the individual APIs, predicting an improved pharmacokinetic behavior for ctc. Together with findings from preclinical studies, these data support the clinical development of ctc for the treatment of pain.
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INTRODUCTION The adequate treatment of pain has not yet been achieved, with only around one in four patients obtaining significant pain relief. This is partly a result of the subjective nature of pain but is also due to poor tolerability and concerns over long-term safety and abuse potential of available analgesic drugs.1 Thus, although pain is the most common symptom for which patients seek medical attention, its management still represents a clear unmet medical need.2 As happens in many areas of medicine when monomodal therapies fail to provide a complete response, combining existing therapies is a common clinical practice in pain management. Indeed, many efforts are expended in performing clinical studies to assess the best combinations of available analgesic drugs.3 The validity of this approach is explained by the multimodal nature of most pain states, in which several different mediators, signaling pathways, and molecular mechanisms are implicated. Complementary recruitment of several mechanisms of action has the potential to provide more effective analgesia, as long as additive or synergistic interactions occur. The use of combination therapy could allow lower doses of the active drugs to be used, potentially reducing the incidence and severity of side effects and thereby improving the efficacy-to-safety ratio.4 To date, three main approaches to such “polypharmacology” have been adopted:5 use of drug cocktails (two agents, two pills), development of multicomponent drugs © 2017 American Chemical Society
(two agents, one pill), and discovery of single molecules with multiple mechanisms of action (one multimodal agent, one pill). Co-crystals incorporating two different active pharmaceutical ingredients (APIs) in the same crystal lattice represent a new alternative approach that lies somewhere between a multicomponent drug and a multimodal agent. A consensus paper defined co-crystals as “solids that are crystalline single phase materials composed of two or more dif ferent molecular and/or ionic compounds generally in a stoichiometric ratio which are neither solvates nor simple salts”.6 If one of the components is an API, then it is recognized as a pharmaceutical co-crystal.7 Such pharmaceutical co-crystals typically comprise a single API crystallized with a pharmaceutically acceptable inactive molecule, called a coformer.8 APIs may also be co-crystallized with other APIs, thereby providing API−API co-crystals and offering a new strategy that can overcome some of the problems associated with traditional fixed-dose combinations (FDCs). Such problems include those encountered in the formulation stages regarding stability, solubility differences, and chemical interactions between the individual APIs.9 By virtue of the unique crystalline structure formed on the basis of weak intermolecular interactions between both drugs, API−API coReceived: December 16, 2016 Revised: February 14, 2017 Published: March 6, 2017 1884
DOI: 10.1021/acs.cgd.6b01848 Cryst. Growth Des. 2017, 17, 1884−1892
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Figure 1. Noncovalent linking approaches to polypharmacology, combining the action of two drugs, A and B, which bind to their respective targets, 1 and 2. (a) Cocktails involve the use of two available drugs. Multicomponent drugs are composed of two active pharmaceutical ingredients (APIs) that can either be in the form of (b) a fixed-dose combination (FDC) or (c) an API−API co-crystal. The three approaches are ordered according to their innovation and potential for property improvement.
ential inhibitor of cyclo-oxygenase-2 (COX-2).17 It is indicated for the relief of chronic pain in osteoarthritis, rheumatoid arthritis, and ankylosing spondylitis18 as well as acute pain and primary dysmenorrhea,19 exhibiting fewer adverse side effects and a lower risk of cardiovascular complications compared with many of the coxib analogues.20,21 Tramadol is a centrally acting weak synthetic μ-opioid receptor agonist, indicated for the treatment of moderate to severe pain. It is a racemic compound, and its analgesic effect is thought to be due to the combined activity of the racemate with further contribution from the O-desmethyl (M1) metabolite, which has higher affinity for the μ-opioid receptor than the parent compound.22 Additionally, the compound inhibits the reuptake of noradrenaline and serotonin and is therefore a multimodal agent. The dextrorotatory enantiomer of tramadol has higher affinity for the μ-opioid receptor and is a more potent inhibitor of serotonin reuptake, whereas the levorotatory enantiomer of tramadol is a more potent inhibitor of noradrenaline reuptake. We recognized that development of a co-crystal between tramadol and celecoxib could have significant therapeutic application if formation of the co-crystal conferred properties that optimized the PK of each therapeutic moiety. This is relevant considering celecoxib is a Biopharmaceutics Classification System Class II drug and practically insoluble in water (7 μg/mL), which makes the development of suitable formulations containing this agent highly challenging. Cocrystallization also represents an opportunity to improve the safety ratio by decreasing the doses of both APIs while maintaining the same efficacy, which is pertinent in view of the US Food and Drug Administration recommendation to use the lowest effective doses of celecoxib.23 Here we report the studies that led to the identification, isolation, characterization, and initial scale-up of a co-crystal of rac-tramadol·HCl and celecoxib in an intrinsic 1:1 molecular ratio (Co-Crystal of Tramadol Hydrochloride-Celecoxib, ctc; E-58425/MR308). We also describe its crystalline structure and dissolution profile, which, together with its robust pharmacological properties, led to the selection of ctc for clinical development.
crystals have the potential to offer improved physicochemical properties, including enhanced solubility and dissolution,10 thereby resulting in improved pharmacokinetics (PK)11 and/or bioavailability,12 relative to FDCs and other combinations. Additionally, co-crystals can facilitate the different stages of formulation development by improving stability13 or tabletability.14 In the clinical setting, advantages may manifest as the provision of a greater therapeutic effect, improved efficacy-tosafety ratio, a reduction in the number of prescriptions and administrative costs, and an increase in patient compliance.15 Figure 1 shows a schematic representation of the three approaches to multimodal therapy based on the noncovalent linking of two APIs, i.e. cocktails, FDCs, or API−API cocrystals. These are listed in order of increasing innovation and potential for property improvement versus the parent drugs. API−API co-crystals are, therefore, novel and unique solid forms which display physical and chemical properties different from their parent APIs. So far, they are relatively scarce, with only a few examples described in the literature.16 In our proprietary pain relief platform, we apply a series of criteria for the application of co-crystal technology to selected compounds, including appropriate molecular mechanisms and sites of action, synergy in pain relief, PK characteristics that can be improved, and absence of deleterious metabolic, pharmacodynamic, or safety interactions between the two APIs. Using this platform, we identified two APIs, tramadol (ractramadol·HCl, 1) and celecoxib (2), to which we applied the co-crystal technology (Figure 2). Celecoxib is a well-known nonsteroidal anti-inflammatory drug (NSAID) and a prefer-
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EXPERIMENTAL SECTION
Materials. rac-Tramadol·HCl (1) and celecoxib (2) were provided by Sigma-Aldrich and Molekula Limited, respectively. All other agents and solvents were purchased from Sigma-Aldrich and were used without further purification. Sample Preparation and Crystallization. A wide co-crystal screening was performed with rac-tramadol·HCl and celecoxib using a number of different methods, including wet grinding, slurrying, evaporation, crystallization, and vapor diffusion.
Figure 2. Structures of rac-tramadol hydrochloride (1) and celecoxib (2). 1885
DOI: 10.1021/acs.cgd.6b01848 Cryst. Growth Des. 2017, 17, 1884−1892
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Wet Grinding. Equimolar amounts of rac-tramadol·HCl (1; 48 mg, 0.16 mmol) and celecoxib (2; 61 mg, 0.16 mmol) with a catalytic amount of a selected solvent were mixed in a 1.5 mL stainless steel grinding jar containing two stainless steel balls and milled at 30 Hz for 30 min in an oscillatory ball mill (Mixer Mill MM200, Retsch GmbH, Haan, Germany). The resulting white powders were analyzed by X-ray power diffraction (XRPD). When methyl isobutyl ketone (MIBK) or isopropyl alcohol (IPA) were used as the solvent, ctc was obtained together with an amorphous phase. ctc Preparation Scale-up. Since wet grinding was not considered suitable for scale-up, crystallization, which is generally preferred due to its easier operational conditions,24,25 was attempted. A preliminary solubility study showed that tramadol·HCl (1) was soluble in water, dimethyl sulfoxide, alcohols, and chlorinated solvents, while celecoxib (2) was insoluble in water and soluble in many organic solvents except the most apolar ones. Among the solvents in which both compounds showed similar solubility (dimethyl sulfoxide, methanol, ethanol, IPA, and dichloromethane), ethanol and IPA were selected, since they are International Conference on Harmonization (ICH) class 3 (guideline 1).26 The initial co-crystallization experiments performed with ethanol and IPA using seeds of ctc afforded the desired co-crystal in both cases, but the high solubility of both components in ethanol made it necessary to use an antisolvent, such as cyclohexane or heptane. Since a monosolvent system was preferred, IPA, in which the two APIs were less soluble, was selected for more detailed study. A complete solubility characterization of 1, 2 and ctc was performed in IPA using the Crystal16 parallel crystallizer (Avantium, Amsterdam, Netherlands). The temperature of dissolution in IPA for each compound was measured using different amounts of solid, and the solubility data of the pure components were fitted to the Van’t Hoff equation27 using the CrystalClear software (Avantium, Amsterdam, Netherlands). IPA was shown to be a good choice since the 1:1 stoichiometry line crossed in the middle of the ctc Ksp curves, indicating that it would not be necessary to use an excess of one of the components in order to stabilize ctc. This allowed use of equimolar amounts of both components without any risk of dissociation during the washing step. The best conditions found were: Hot IPA solution of 1 (176 mg/ mL, 0.59 mol) added dropwise over an IPA solution at reflux of 2 (224 mg/mL, 0.59 mol). The resulting solution was seeded (with a sample obtained in the wet grinding method) and cooled slowly to room temperature. Crystallization was very progressive, starting only from 60 °C. The mixture was maintained for 30 min at 60 °C before cooling to room temperature and then to 0−5 °C to give ctc in 91% yield. Preparation of ctc for Single-Crystal X-ray Diffraction (SCXRD). Equimolar quantities of rac-tramadol·HCl (1, 40 mg) and celecoxib (2, 52 mg) were dissolved in a mixture of IPA/heptane (1:1, 4 mL) at 50 °C before slowly cooling to room temperature. Good-quality single crystals of ctc suitable for X-ray diffraction studies were obtained after 3 days by slow crystallization at room temperature. Characterization of Co-crystal. Proton Nuclear Magnetic Resonance (1H NMR). 1H NMR was performed in CD3OD in a Varian Mercury 400 spectrometer, equipped with a 5 mm 1H/19F/X automated triple broadband probe. Infrared (IR) Spectroscopy. Fourier transform IR (FTIR) spectra were recorded using a Thermo Nicolet Nexus 870 FT-IR spectrometer, equipped with a beamsplitter KBr system, a 35 mW He−Ne laser as the excitation source, and a deuterated triglycine sulfate (DTGS) KBr detector. The spectra were acquired in the 600− 3600 cm−1 range at 32 scans at a resolution of 4 cm−1. The samples were prepared using the KBr pellet method. Differential Scanning Calorimetry (DSC). DSC was performed on a Mettler DSC822e (Mettler Toledo, Leicester, UK) with a scan range of 30−200 °C, at a scan rate of 10 °C/min, and nitrogen purge of 50 mL/min. Thermogravimetric Analysis (TGA). TGA was recorded in a thermogravimetric analyzer Mettler TGA/SDTA851e. Each sample was weighed into a 70 μL alumina crucible with a pinhole lid and heated at 10 °C/min from 30−200 °C, under nitrogen (50 mL/min). X-ray Powder Diffraction. XRPD patterns were collected in reflection θ−θ geometry on a PANalytical X’Pert PRO diffractometer,
equipped with Cu Kα radiation (λ = 1.5406 Å) and a PIXcel detector, operated at 45 kV and 40 mA. The measurement angular range was 3.0−40.0° (2θ) with a step size of 0.013°. The scanning speed was 0.164°/s (20.40 s/step). Single-Crystal X-ray Diffraction. Unit cell and intensity measurements were carried out on a Bruker SMART APEX CCD area-detector diffractometer, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data were collected using SMART (Bruker AXS Inc., Madison, WI, USA). Cell refinement and data reduction were performed with the program SAINT (Bruker AXS Inc., Madison, WI, USA). Systematic absences were consistent with space groups P21nb (No. 33) and Pmnb (No. 62); the former was chosen on the basis of the value of Z (the formula cannot have internal symmetry); transformation to the standard setting (Pna21) was then performed. The structure was solved by direct methods and refined on F2 by the full-matrix least-squares method using SHELXTL-NT (Bruker AXS Inc., Madison, WI, USA).28 A mixed treatment of hydrogen atoms was adopted. Carbon-bonded H atoms were introduced in calculated positions and refined as riding on their parent atoms with idealized geometries and with restrained isotropic displacement parameters. H atoms bonded to oxygen and nitrogen atoms were refined using isotropic displacement parameters at the positions found in difference Fourier maps. Attempts to model the disorder observed in several fragments were made, and geometrical restraints were applied to the disordered CF3 group. Distances of the phenyl ring of the tramadol molecule were also restrained. Drawings of the structure and the simulation of the powder diffractogram from the single-crystal data were performed with the program Mercury (Cambridge Crystallographic Data Centre, Cambridge, UK).29 Intrinsic Dissolution Rate (IDR). Dissolution tests were performed on ctc in comparison to 1 and 2 as described in US Pharmacopeial Convention USP .30 The pellets of the drug substances were prepared applying a pressure of 1200 psi for 2 min with an intrinsic dissolution surface device from International Crystal Laboratories (Garfield, NJ, USA; plate ref 0012-8102; punch ref 0012-8103). Changes in the original solid forms on pellet formation were discarded by XRPD. The dissolution tests were carried out with a USP 2 dissolution tester (Varian VK7010; Varian Inc., Cary, NC, USA) in 1000 mL of water at 37 °C and a rotation speed of 50 rpm. Sampling time was every 15 s when testing for tramadol·HCl dissolution and every 60 s when testing for celecoxib dissolution. Oversaturation Studies. Oversaturation studies of the less soluble component celecoxib (2) were performed on ctc in comparison to an equimolar blend of pure 1 and 2. The samples of ctc and blend of pure 1 and 2 were milled in an agate mortar to homogenize particle size and thereby avoid particle size effects. The dissolution rate of celecoxib (2) was measured in nonsink conditions with a USP 2 apparatus (Hanson SR8 Plus, Hanson Research Corp., Chatsworth, CA, USA) using 100 mg of substance in 1000 mL of water at 37 °C and a rotation speed of 50 rpm. Samples were withdrawn through 70 μm polyethylene filters and analyzed by high performance liquid chromatography. The sampling rate was every 15 min in the first hour and every 30 min from the second to the fourth hour. rac-Tramadol·HCl is a very soluble substance, and no oversaturation was observed for this component.
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RESULTS AND DISCUSSION Obtaining a co-crystal of tramadol·HCl (1) and celecoxib (2) was foreseen as a challenging, nontrivial task. When this research was initiated, no reports on co-crystal formation with tramadol were available; for celecoxib, only two co-crystals, with the coformers nicotinamide and 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane), had been reported.31,32 Moreover, the physicochemical characteristics of both drugs are quite different: celecoxib is a highly insoluble molecule with no ionizable group at physiological pH, while the commercially available form of tramadol is a hydrochloride salt that is much more soluble than celecoxib and possesses a 1886
DOI: 10.1021/acs.cgd.6b01848 Cryst. Growth Des. 2017, 17, 1884−1892
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bic space group Pna21 with one moiety of tramadol·HCl and one molecule of celecoxib in the asymmetric unit (Figure 3a). As in rac-tramadol·HCl, the structure of ctc contains both enantiomers of tramadol in a 1:1 ratio. The hydrogen bonding scheme and a view of the packing of the structure are shown in Figure 3b and 3c, respectively. In the ctc structure, adjacent celecoxib molecules, related by the a glide planes, are joined through chloride bridges by means of N−H···Cl− hydrogen bonds involving both hydrogen atoms of the NH2 group (Cl1···N4(H4A), 3.263(4) and Cl1··· N4(H4B), 3.278(4) Å), to form an infinite chain (C12(4))36 along the crystallographic [100] direction (Figure 4). The chloride ions form a third hydrogen bond, N+−H···Cl−, in this case with the tertiary amine of the tramadol molecule (Cl1··· N1(H1), 3.073(3) Å). Moreover, tramadol and celecoxib molecules are also directly linked through O−H···OS bonds (O3···O2(H7), 2.849(3) Å). These two last links, together with one of the previously mentioned N−H···Cl− contacts (Cl1··· N4(H4B)), form chains (C23(12)) along the crystallographic [011] and [011] directions (Figure 5), indefinitely extended by the n glide planes. The crossing of all these chains results in a 3D network, with rings (R127(36)) parallel to the crystallographic (011) and (011) planes. This network is defined by an extensive hydrogen bonding scheme, where chloride ions play a key role and are linked by hydrogen bonding to tramadolium ions. The analysis of the supramolecular synthons37 involved in ctc shows that it is a rather original structure, since no crystal structures containing at the same time synthon ctc-II and the synthons involving chloride ions ctc-I and ctc-III (see Figures 6a and 7a) have been found in the literature. When looking at the published crystalline structure of pure rac-tramadol·HCl (BUFPAV),38 it can be observed that chloride ions are linked to tramadolium molecules by two kinds of hydrogen bonds: (−N+H···Cl−) and (−OH···Cl−), forming a C12(8) motif, also observed in the ionic co-crystal of rac-tramadol·HCl with benzoic acid (AYUNUE).39 In this structure, a third hydrogen bond with the carboxylic acid group of benzoic acid is then formed, producing synthon I (Figure 6b), according to the fact that carboxylic acid is a good functional group to form hydrogen bonds with chloride ions.40 In the co-crystal of tramadol and paracetamol,41 hydrogen bonding with the chloride ion is produced via a water molecule (synthon II, Figure 6c), as often encountered in the literature.42 In the case of ctc, the C12(8) motif is not observed. The chloride ion forms an N+H···Cl− link with one tramadolium molecule, and the two other possible links of chloride ions are formed with celecoxib sulfonamide (ctc-I; Figure 6a), breaking the tramadol·HCl chain and forming a celecoxib···Cl− chain C12(4). Regarding ctc-II (Figure 6a), this synthon is more anticipated, since, for example, it has been described in some solvates (such as BOKHIU, BOKHOA, BOKHUG, BOKJAO),43 a clathrate (WOPCIO),44 and some monocomponent structures (such as KUVFIR45 or NIGNEY46). Regarding the celecoxib synthons, ctc is quite unique, since it is the only celecoxib co-crystal containing a hydrochloride salt. The chloride ions link the celecoxib molecules, forming a hydrogen bond with sulfonamide groups (ctc-III Figure 7a). Few additional examples of celecoxib co-crystals have been described: with nicotinamide and 18-crown-6,31,32 with different coformers, namely those with L-proline,47 aromatic nitrogen compounds,48 or cyclic lactams49 and also with venlafaxine,50
positive ionizable group within the physiologically meaningful pH range. A thorough screening of different techniques for obtaining co-crystals such as wet grinding, slurrying, evaporation, and vapor diffusion, using various classes of solvents, was undertaken before co-crystallization of 1 and 2 was achieved (for a review on co-crystal preparation, see Qiao et al. 201124). Only after wet grinding with MIBK or IPA was a co-crystal detected, mixed with an amorphous phase. Using this solid as seeding, ctc (E-58425/MR308), consisting of rac-tramadol·HCl and celecoxib in a 1:1 molecular ratio, was isolated by crystallization in IPA/heptane.33,34 ctc was unambiguously characterized by SCXRD, XRPD, NMR, IR, DSC, and TGA. The compound can be classified as an ionic co-crystal,35 which is defined as that typically sustained by charge-assisted hydrogen bonds and normally formed by a positive moiety, a negatively charged counterion, and a neutral molecule. Single-Crystal Structure Analysis. Crystal data, intensity data collection parameters, and final refinement parameters are summarized in Table 1. Table 1. Crystallographic Data and Structure Refinement for ctc ctc C33H40ClF3N4O4S 681.20 0.33 × 0.16 × 0.11 colorless prism 294(2) orthorhombic Pna21, 4 11.0323(7) 18.1095(12) 17.3206(12) 90 90 90 3460.5(4) 1.308 0.71073 0.229 1.63 29.12 18327 8336 5226 >2σ(I) 0.0585 0.1109
Empirical formula Formula weight Crystal size (mm3) Color, habit T (K) Crystal system Space group, Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) ρcalcd (g/cm3) X-ray wavelength (Å) μ (mm−1) θmin (deg) θmax (deg) No. of reflections collected No. of independent reflections No. of observed reflections Threshold expression R1 (observed) wR2 (observed)
The hydrogen bond parameters are depicted in Table 2. The new entity crystallizes in the noncentrosymmetric orthorhomTable 2. Hydrogen Bond Geometries (Å, deg) of ctc
a
DH···A
d(DH)
d(H···A)
d(D···A)
