Varying linker length in poly(pro-17β-estradiol) affects mechanical properties, thermal properties, and release kinetics

Abstract

The female sex hormone, 17β-estradiol (E2), possesses neuroprotective, neurotrophic, and anti-inflammatory properties. In its native state, E2 cannot reach its full potential. However, modification of E2 into a prodrug increases the scope of E2’s applications. To harness these qualities, a polymerized prodrug of E2, poly(pro-E2), was synthesized, in which E2 is a component of the polymer backbone which can be released as the active drug upon hydrolysis. Incorporation of E2 into a poly(prodrug) was executed to create a biomaterial capable of undergoing sustained drug release.In this research, synthesis and characterization of three variants of poly(pro-E2) polymers (PE-C3, PE-C6, and PE-C9) were performed. These polymers vary in their carbon linker length, containing a single saturated 3, 6, or 9–carbon chain in the backbone. It was hypothesized that the addition of longer flexible chains would increase the polymer’s flexibility, which was confirmed through characterization techniques, such as nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Additionally, the degradation rates and kinetic drug release profiles of the poly(pro-E2) films were tracked over the course of months. These novel polymers were processed into solvent cast films, and subsequently incubated and degraded via hydrolysis. Release kinetics were quantified through an enzyme-linked immunosorbent assay (ELISA). The PE-C9 variant exhibits the greatest flexibility, characterized by a low glass transition temperature (Tg) and reduced moduli. This behavior stems from longer sections of flexible segments between rigid regions which expand the system’s free volume and enhance chain mobility. While thermal and mechanical properties align in order with linker length, release kinetics do not: PE-C6 demonstrates the highest release rate after 16 weeks, despite occupying a median position regarding polymer flexibility. This discrepancy may be explained by two countervailing effects between these variants: polymer flexibility and hydrophobicity. In PE-C9, the longer alkyl linker increases free volume, which would theoretically allow for greater water uptake and hydrolysis. However, the increased carbon density simultaneously increases the hydrophobicity of the matrix, inhibiting water intake. We conclude that PE-C6 strikes an optimal balance between these two properties, leading to a significantly greater E2 release compared to its C3 and C9 counterparts. Ultimately, this work aims to develop a biomaterial capable of demonstrating slow, sustained, localized release of a neuroprotective and neurotrophic drug that can be utilized in central nervous system (CNS) injury recovery and various other medical applications.