Thermosensitive hydrogel system for sustained release of estradiol in a fish model

Introduction Despite the fact that sustained administration of hormones has been shown to be effective in aquaculture, at the present time, only minimal research is devoted to this issue and only a few sustained release systems have been developed and tested on fish breeding, such as cholesterol pellets implants, biodegradable microspheres, non-degradable monolithic implants and more recently, osmotic pumps. Although these systems have generally demonstrated a good performance when administrated to fish, the production and wider distribution on an industrial scale appears improbable due to the high production cost and to the difficult administration. The use of a parenteral thermogelling systems has previously been shown to be effective for controlled release of hormones when applied in different animal species. These formulations increased the relative bioavailability of steroidal hormones and resulted in the sustained release of testosterone after a single subcutaneous (s.c.) injection in rabbits. Aim of the work The purpose of this study was to develop and evaluate a delivery system comprising a thermosensitive gel for the sustained release of 17β-estradiol (E2) in fish, over an extended period of time after a single intramuscular (i.m.) injection and for the improved reproductive performance in fish. Experimentals Poloxamer 407 (LUTROL F-127) was used to formulate the thermosensitive gelling system. Rheological analysis was performed using (Stress-Tech, Reologica Instruments, Lund, Sweden) equipped with a cone-plate geometry (4/40). In vitro release of E2 was evaluated using an USP dissolution apparatus 2 (AT7 smart, Sotax, CH) equipped with teflon Enhancer Cells (Agilent, USA). In vivo studies have been performed on goldfish (Carassius auratus), analyzing the plasma concentration of both E2 and vitellogenin (VGT) with Enzyme-immunoassay (EIA) and ELISA techniques. Results and discussion The poloxamer 407 based formulation exhibited a gel point at 20°C and behaved as a Newtonian low viscosity system at temperature lower than 15°C. The results demonstrate that a single i.m. injection of the thermogel formulation containing E2 can increase relative bioavailability of the hormone resulting in prolonged presence of high plasma VTG levels. E2 and VTG levels after i.m. injection of gel-E2 remained high for more days compared to the duration observed following both oil-E2 (this study) and double E2 injection experiments. Overall, the observed levels are comparable to those of mature vitellogenic female goldfish. Conclusion This work demonstrates that the poloxamer 407 thermogelling system show the potential to work as a long-acting sustained release vehicle for steroidal hormones in fish. The fast preparation, the low cost of the main formulation components, the high versatility, together with the much easier administration compared to other prolonged release systems currently studied such as implants, make these systems particularly promising in the field of hormonal treatment in aquaculture.