Novel Long Term Substance Delivery Method in Chronic Dosing Studies
Authors: Rissanen Jukka , Ruohonen Saku , Jukarainen Harri 
Published: November 2023

IntroductionChronic dosing of compounds often has pivotal role in preclinical efficacy and toxicology studies. A vast number of dosing methods have been applied to pursue the cost‐efficient, ethical, stable and long‐term release of compounds. However, current dosing methods are not applicable for long‐term substance delivery in animal studies or release is unstable. One class of widely used chronically dosed compounds are hormones and other steroid‐structured compounds. We have developed a cost‐effective, adjustable and sustained polymer‐based substance delivery matrix named as MedRod™, which can provide stable drug release from weeks to several months and even years in either in vivo or in vitro studies. MedRod™ is optimal for steroidal compounds. The delivery system is elastic, and it utilizes biocompatible, a biostable soft matrix that remains unchanged in tissue. It is removable at any time of the study if sudden termination of drug delivery is desired.ObjectivesTo study the in vitro and in vivo release of dihydrotestosterone (DHT) and 17b‐estradiol (E2) from MedRod™ matrix.MethodsDHT and E2 were embedded in the polymer matrix and various doses of 3mm OD rods were produced. The release of hormones was studied first in dissolution in vitro test and then in animal studies with mice and rats. Chromatography and ELISA based DHT and E2 immunoassays were performed to quantify hormone levels from the dissolution or serum samples. The macroscopic evaluation was conducted during necropsy and sensitive tissues for hormone metabolism, such as prostate, uterine, liver and bone were harvested and weighed. Moreover, trabecular bone mineral density (BMD) was measured from tibias by peripheral quantitative computed tomography (pQCT).ResultsThe initial burst release characteristic to polymers was mild and observed within the first days of experimentation in both in vitro and in vivo studies, which after the overall release profiles of DHT and E2 were stable for several months. In the animal experiments, the DHT levels in male mice exceeded significantly the endogenic DHT levels in control animals. In the study, DHT exposure induced anabolic effects in hormone‐sensitive organs (prostate, liver, bone)Correspondingly, E2 levels in female rats were elevated during the study and the increase in uterine weights correlated significantly with the elevated E2 levels. Furthermore, increased trabecular BMD confirmed the anabolic effect of the hormone treatment. Liver weights were slightly elevated after hormone treatment in response to increased metabolisms, but during necropsy no evidence of toxicity were observed in either E2 or DHT‐treated animals. In addition, no irritation or inflammation was found from implant surrounding tissue, neither did any of the animals scratch the implanted site, thus proofing comfortable and safe delivery system.ConclusionThe results demonstrate the benefits of using MedRod™ polymer‐based substance delivery in preclinical animal studies. The MedRod™ system enables long‐term and stable substance delivery. Benefits include decreased handling stress for the animals and decreased variability in results with lower costs. The decreased stress and increased accuracy follows closely the 3R principle (Replacement, Reduction, and Refinement).