http://www.e-polymers.org e-Polymers 2005, no. 007. ISSN 1618-7229 Calorimetric investigation of the phase behaviour of the binary system 7-mPEG 5000-succinyloxymethyloxy- carbonyl-Paclitaxel (PP7)/water Dedicated to Dr. E. A. Hemmer, Professor emeritus of the Gerhard-Mercator University Duisburg, Germany, on the occasion of his 75th birthday Jung-Sun Sohn 1, Soo-Kyung Choi 1, Byung-Wook Jo 1, Kirsten Schwark 2, Michael Hess 2 * 1 Department of Polymer Engineering and Science, and Department of New Materials in Biology, Chosun University, 375, Seosok-dong, Dong-gu, KwangJu, 501-759, South Korea; [email protected] 2 Department of Physical Chemistry, University Duisburg-Essen, Campus Duisburg, 47048 Duisburg, Germany; [email protected] (Received: October 22, 2004; published: February 2, 2005) This work has been presented at the 12th Annual POLYCHAR World Forum on Advanced Materials, January 6-9, 2004, in Guimaraes, Portugal Abstract: Paclitaxel is an antineoplastic agent derived from the bark of the Pacific Yew Tree (Taxus brevifolia). Oral administration of the pure drug is problematic as it has a poor absorption due to its poor solubility in aqueous media. A specific water-soluble prodrug – PP7 – has been introduced by coupling the drug with a water-soluble polymer. For any kind of medical application and administration, knowledge of the binary isobaric phase-diagram is important since it gives infor- mation about solubility, phase transitions and the corresponding compositions. The system PP7 + H2O was studied calorimetrically from -20°C and shows the typical behaviour of an eutectic system. The properties of the phase diagram are discussed in view of other solution properties of the system presented earlier. 1. Introduction and scope The application of pharmaceutically active substances is frequently faced with problems of solubility, stability and bioavailability of the active species. In the case of the anti-cancer drug Paclitaxel [1] (Fig. 1) – an antineoplastic substance that can be isolated from the bark of the Pacific Yew Tree (Taxus brevifolia) – the first problem is its hydrophobicity. Despite several polar sites the molecule is only poorly soluble in aqueous media. Also, the pump function for Paclitaxel of the multi-drug pump trans- porter P-glycoprotein of the gastrointestinal tract is poor. There are several ways to overcome this problem. One possible way is substitution [2,3] at the reactive sites on carbons 2' or 7 (Fig. 1). The first such derivatives of Paclitaxel with poly(oxyethylene) 1 (PEO), however, were too stable so that a high percentage of the polymer drug was excreted before the drug could be made bioavailable by esterases on-site of their intended interaction. This problem was solved by the development of the prodrug PP7, which connects the hydrophilic PEOS – a succinic acid monoester of PEO monomethyl ether – with a self-immolating linker [4-7] at carbon 7 (Fig. 1). Some results about properties of PP7 in aqueous solution have been published earlier [8-10]. The term ‘prodrug’ indicates that PP7 is not the finally active species. Enzy- matic cleavage of the bond with the polymer substituent is necessary so that the Paclitaxel molecule can interfere in the late phase of G2 mitosis by stabilising the microtubule through non-covalent interactions. The thermodynamic properties of the system PP7/H2O, cumulated in the isobaric phase diagram, give important information about different aspects of the system, e.g., when the physical properties or the polymers themselves have to be characterised. Many of the experimental techniques can only be performed in homogeneous solution, and knowledge about the state of the polymer drug is essential for the method of its application and bioavailability. 7 2' Fig. 1. Structure of Paclitaxel with the substitution sites 2' and 7 The state of the prodrug is also important for its capability to interfere with malign tissue. Here it is of particular importance that the PEO chains – depending on their molar masses – show affinity to certain tumour types [11]. Finally, the phase diagram of the polymer-solvent system contains information about interaction parameters and thermodynamic equations of state and gives useful hints for the alteration of the molecular structure of the water-soluble drug in order to cause certain behaviour in aqueous solution like complex formation and aggregation, which might me desired to increase the local concentration of the drug in solution. Know- ledge of the phase behaviour of pharmaceutical systems is therefore of great impor- tance far beyond just academic interest. The investigation of the phase diagram is one more step on the way to understand the mechanism of drug activity with respect to conformation, shape and size in solution, transport mechanisms, and interaction with other constituents of body fluids. 2 2. Experimental part 2.1 Materials Paclitaxel obtained from Brystol-Myers Squibb Co., NY, USA, was modified with α- methyl-terminated ω-OH-functional PEO (Fluka) (〈Mw〉/〈Mn〉 = 1.05 and molar mass averages of 5000 g/mol) and linked to carbon 7 (see Fig. 1) with succinic acid through a self-immolating linker. The modified drug (the prodrug) is termed PP7 (5000), the α-methyl- and ω-succinyl-terminated PEO is termed PEOS in the following text. The additive ‘-5000’ indicates the molar mass of the PEO since also other degrees of polymerisation can be used. However, the yield decreases with increasing length of the polymer chain. The synthesis of the prodrug has been described elsewhere [4-7,12]. 2.2 Methods Calorimetric measurements were carried out with a Perkin-Elmer DSC-7 differential scanning calorimeter. A total mass of approximately 20 mg was placed in a sealed pressure pan and heated up to 80°C for complete solution. After keeping the solution at 80°C for about 5 min, the sample was cooled down to -20°C with a cooling rate of 20 K/min and the glass transition temperature of the pure PEO (〈Mw〉= 5000 g/mol) at -63°C was not reached. The samples were heated with a heating rate of 20 K/min. Cold crystallisation during heating was not observed. Fig. 2. Construction of a phase diagram from DSC experiments The phase diagram was constructed according to Fig. 2 by taking the end-point of the first-order transition, which is supposed to be close to the equilibrium transition temperature since the largest crystals of the polymer melt at that temperature. The classification of thermodynamic transitions follows Ehrenfest [13]. Within the temper- 3 ature range of observation no cold crystallisation was observed. The crystallisation of non-equilibrium structures usually occurs at temperatures closer to the glass tran- sition (around -40°C for PEO) [14]. 3. Results and discussion A phase diagram shows thermodynamic transitions depending on the composition of the system given as the volume fraction or the mass fraction at constant pressure. A typical example of a binary system is given in Fig. 3. In most cases, phase diagrams show equilibrium situations only. However, it has been argued [15] that non-equi- librium but long-living phases deserve to be included in phase-diagrams, thus providing more information. Fig. 3. General scheme of a common phase diagram. Further explanations see text Region I is the homogeneous isotropic liquid mixture (one phase) of components 1 and 2. Regions II and III represent solid one-phase regions consisting of mixed crystals rich in component 1 (II) or rich in component 2 (III), respectively. These two phases can be very narrow or even absent. Sometimes these regions are addressed as representing a ‘border solubility’. Regions II and III vanish when the liquid phase I is in equilibrium with a solid pure component 1 or 2, respectively. The two solid one- phase regions II and III are separated by the solid two-phase region VI. At temper- atures between Te and Tm,1 or Tm,2, the solid one-phase region II is separated from the liquid one-phase region I by the two-phase region IV, and the liquid one-phase region I is separated from the solid one-phase region III by the 2-phase region V. As a consequence of Gibbs phase rule two one-phase regions have always to be separated by one two-phase region. Inside the two-phase regions the lever rule is valid, e.g., between the points a and a' and b and b'. The curves Tm,1 – e - Tm,2 – e (liquidus curve), Tm,1 – c - Tm,2 – d (solidus curve), c – c', and d – d' ( solvus curve) are equilibrium curves. e is the eutectic point and has no degree of freedom. c – e – c' is a 4 monovariant line. The other lines are bivariant. The lines can continue within the phases as indicated by the dotted lines describing metastable states. The solid lines describe the coexistence curves of the thermodynamic equilibrium state. In a real solid-liquid equilibrium system, e.g., a melt with the composition x2,a is in thermodynamic equilibrium with mixed crystals of composition x2,a' at the temper- ature Ta. Thermodynamic melting equilibrium requires infinitely large crystals. In polymers and in dynamic measurements this situation is approximated at a low heating rate and at the end of the DSC melting curve. Systems containing a solvent and a polymer have always to be considered as quasi-binary ones because of the non-uniformity of polymers. Fig. 4. Examples of DSC traces of mixtures of PP7 (5000) + H2O of different compo- sition. The melting transition of the PEO chain is clearly identified at higher temper- atures. The melting of water and the invariant temperature of the solidification temperature of the eutectic mixture follow with decreasing temperature. Boundary solubility cannot unequivocally be identified.