Encapsulation of all-trans-retinoic acid in polymeric micelles

All-trans-retinoic acid (ATRA) is being increasingly included in antitumor therapeutical schemes for the treatment of various type of tumors; however the difficulty of increasing plasma concentrations of ATRA in vivo hindered, until now, a correct evaluation of its possible pro-apoptotic effect towards both the proliferating and non proliferating neuroblastic cells. Recent approaches to enhance the solubility of hydrophobic drugs considered polymeric micelles particularly interesting for their efficiency in trapping lipophilic drugs within their internal core, the possibility to provide small and flexible structures able to easily extravasate in the solid tumor and a propensity to evade scavenging by the mononuclear phagocyte system. Our studies focus on micelles prepared from a water soluble polymer, PVA, conjugated with lipid alkyl chains, thus achieving amphiphilicity, such as that of A-B type block co-polymers. The core of the polymer serves as a non-aqueous reservoir, where the drug is entrapped, but also stabilized against chemical modifications by the physiological environment [1]. All-trans-retinoic acid (ATRA) is now included in many anti-tumour therapeutic schemes. Unfortunately its poor aqueous solubility hampers its parenteral formulation. To date, there is no parenteral formulation of ATRA commercially available, while oral administration of ATRA is associated with progressively diminishing ATRA levels in plasma. An ATRA formulation, obtained by complexation of the drug into polymeric micelles, might be suitable for parenteral administration overcoming these unwanted effects [2]. To this purpose we prepared an amphiphilic polymer by poly-vinylalcohol (PVA) substitution with oleyl amine at 1.5% substitution degree and evaluated its functional properties with regard to ATRA complexation. EXPERIMENTAL PART The new amphiphile polymer, PVA-co-oleyl-vinyl carbamate, obtained by PVA partial substitution with oleylamine, contains a carbamate bond, rather than an ester bond, as a spacer between the hydrophilic PVA and the hydrophobic oleyl chains. The polymer thus obtained holds enhanced aqueous solubility in the presence of enhanced substitution degree. The solubilizing ability of the substituted polymer was evaluated towards ATRA in aqueous solutions of the polymer and of drug–polymer mixtures obtained by spray-drying hydro-alcoholic solutions of ATRA and polymer at 1:3, 1:5 and 1:10 drug–polymer weight ratios. RESULTS AND DISCUSSION The spray-dried complexes rapidly dissolve in water, providing high levels of ATRA solubilization as a function of the drug–polymer weight ratio. The complexes characterized by 1:5 drug–polymer weight ratio (▲) provided higher levels of ATRA solubilization than 1:3 (▄) and 1:10 (▬) drug–polymer weight ratios respectively. Pre-formed polymeric micelles in water, equilibrated in the presence of excess solid ATRA, provided lower levels of solubilization (●). The drug release from the complexes was very slow in PBS, indicating their suitability in anti-tumour drug targeting, where a fundamental requirement is stability towards drug release for at least 24 h, corresponding to the average circulation time period of macromolecular carriers. A comparison between the present polymer, containing a polyvinyl backbone linked to oleyl chains by carbamate bonds and a previously studied polymer, based on the same polyvinyl backbone linked to oleyl chains by ester bonds, indicates that the nature of the bond between the main backbone and the oleyl chains strongly influence the physico-chemical characteristics of the final polymer. In the presence of the ester bond the highest polymer aqueous solubility obtained was 30 mg/ml at 37°C at a substitution degree not exceeding 1%. The increase in the substitution degree decreased the polymer solubility. In the presence of the carbamate bond it was possible to increase both the substitution degree and the aqueous pol...