Sustained release of recombinant human insulin-like growth factor-I for treatment of diabetes

Abstract

Recombinant human insulin-like growth factor-I (rhIGF-I) was found to improve glycemic control and enhance insulin sensitivity in patients with a syndrome of severe insulin resistance. Therefore, the protein may be considered as an alternative therapy in the treatment of diabetes when the patients become insensitive to insulin treatment. Because the protein was administered twice per day in the clinical trials, a sustained release polylactic-co-glycolic acid (PLGA) formulation for rhIGF-I with low initial burst (<20%), maximum possible protein loading (15–20%) and a continuous release of 1–2 weeks may provide greater patient convenience and compliance. The protein was encapsulated in PLGA for sustained release using a spray freeze-drying technique. Formulation parameters such as protein loading, polymer end group, and the presence of zinc carbonate were studied for their effects on in vitro release of rhIGF-I from PLGA microspheres. As the protein loading was increased, the initial burst increased. Due to the hydrophilic properties of the polymers, rhIGF-I encapsulated in unblocked PLGA (free acid end groups) gave a lower initial burst and a more steady-state release profile than the blocked PLGA (hydrocarbon end groups) with the same protein loading and PLGA molecular weight. At 15% w/w protein loading, the addition of 6% w/w zinc carbonate as a protein release modifier to the unblocked PLGA (12 kDa) decreased the initial burst of rhIGF-I. Therefore, a formulation consisting of 15% rhIGF-I and 6% zinc carbonate in 12 kDa, unblocked 50:50 PLGA can provide the required release characteristics in vitro. Rat studies revealed that rhIGF-I in this formulation was released in vivo at a rate which was comparable to that observed in vitro. These studies demonstrate the potential for a sustained release, 14-day formulation for rhIGF-I.

Introduction

Insulin-like growth factor-I (IGF-I) has been widely investigated as a potential therapy for the treatment of diabetes due to its metabolic actions similar to that of insulin. The hormone, a homologue of proinsulin, is a single-chain polypeptide of 70 amino acids with three disulfide bonds [1], [2]. In vivo, IGF-I is responsible for mediating some of the growth-promoting effect of growth hormone [3], [4]. Researchers have found that recombinant human IGF-I (rhIGF-I) can improve glycemic control and enhance insulin sensitivity in patients with a syndrome of severe insulin resistance [5], [6], [7]. In addition, rhIGF-1 was also found to improve glucose and lipid metabolism in patients with noninsulin-dependent (Type II) diabetes mellitus [8], [9], [10]. Although the efficacy and mechanism of action of rhIGF-I in the regulation of insulin and blood glucose levels in these patients are still unclear, the protein may be considered as an alternative therapy in the treatment of diabetes when the patients become insensitive to insulin treatment. Currently, patients with Type I diabetes are treated with daily injections of insulin throughout their life time. For Type II diabetes, patients require two to three doses of insulin per day. If rhIGF-I is successfully demonstrated in clinical trials to be a useful adjunct or primary therapy for treatment of diabetes, it would be most beneficial to patients to have a sustained-release form of rhIGF-I so that the administration of the drug twice or more per day can be avoided. Therefore, microencapsulation of rhIGF-I for sustained release would be highly desirable if the protein could be delivered in a single injection on a weekly or biweekly basis.

Microencapsulation of recombinant proteins for controlled release has been successfully performed with human growth hormone (rhGH), interferon-γ (rhIFN-γ), interleukin-2, and MN rgp120 [11], [12], [13], [14], [15]. The sustained-release formulations of these proteins were developed using polylactic-co-glycolic acid (PLGA) due to its biocompatibility and wide range of biodegradable properties. The degradation products of PLGA, lactic and glycolic acids, can be cleared quickly within the human body. Moreover, the residence time of this polymer in vivo can be adjusted from weeks to years by varying the molecular weight and composition [16].

The purpose of this study was to encapsulate rhIGF-I within PLGA microspheres to produce a sustained-release formulation with release characteristics suitable for the treatment of diabetes. Based on the efficacious results from the Phase II clinical studies and the maximum possible dosing of rhIGF-I in a sustained-release form, a formulation that can provide a maximum dosing of 80 μg/kg per day would be desirable. Therefore, for patients with maximum body weight of 85 kg, the largest dosing would be 6.8 mg rhIGF-I per day. In order to achieve this dosing level, a sustained-release formulation which contains a maximum possible protein loading (15–20% w/w rhIGF-I) with the lowest possible initial burst (<20%) is necessary. An initial burst is the rapid release (within a few hours) of protein at or near the surface of the microspheres. Due to safety concerns, high peak levels of protein as a result of initial burst from sustained-release drug products should be avoided. A continuous (zero-order) release of rhIGF-I from microspheres for 1–2 weeks is also desirable. In addition, the encapsulated protein to be released should maintain its integrity and stability over the desired release period. Various microencapsulation process parameters and in vitro release effects were also studied to develop a sustained-release formulation of rhIGF-I with the desired release characteristics to treat patients with diabetes.