Previous research into long-acting drug delivery platforms focused on peptides, short chains of amino acids that can be easily modified at the molecular scale. The success of these peptide-based systems, however, was limited by their biostability. “When the solution is injected, enzymes trigger the formation of a hydrogel depot within the subcutaneous skin space that releases the drug at clinically relevant concentrations over an extended period,” explains Garry Laverty, Associate Professor at Queen’s University Belfast. “The problem with the peptide-based hydrogel is its tendency to break down rapidly in vivo, resulting in the release of inappropriate drug levels.”
In order to improve biostability, Laverty initially focused on the development of a sequence based predominantly on peptoids (peptide-like synthetic molecules with a modified chemical structure). “The advantage of peptoids is that they’re cheaper and easier to synthesise than other peptide alternatives, thus increasing the potential for future large-scale manufacture and clinical translation of the developed system,” explains Laverty. With funding from the Wellcome Trust, the ability of the sequence to form hydrogels was optimised by the sequential addition of peptide molecules. “Rapid gel formation is very important as the hydrogel acts as a barrier, slowing diffusion of the drug and allowing it to be delivered to clinically relevant concentrations for longer,” explains Laverty. The approach led to the discovery of a peptoid-peptide template that forms a biostable drug-releasing hydrogel depot in response to phosphatase enzymes present in skin.
The development of this peptoid-peptide hybrid as a long-acting drug delivery platform for HIV/AIDS was explored with funding from the UK’s Engineering and Physical Sciences Research Council (EPSRC). The antiretroviral zidovudine was selected as a model low-molecular-weight drug and attached to the peptoid-peptide hybrid. Microscopic examination of the peptoid-peptide hydrogels was first carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) at Queen’s University Belfast. “The hydrogel is composed of a network of fibres and these imaging techniques revealed a three-dimensional interweaving random entanglement of fibres,” explains Laverty.
Small angle neutron scattering (SANS) is capable of revealing additional complementary information about the structural features of fibrous networks. Knowledge about the capabilities of neutron techniques was introduced to Laverty’s group by the arrival of Emily Cross as a postdoctoral researcher, following a PhD with Professor Dave Adams at the University of Glasgow, a regular user of neutrons in collaboration with Ralf Schweins, instrument scientist at the ILL. “SANS provides information at the 1−100 nm scale about how the molecules are packed within a hydrogel sample,” explains Schweins. “Furthermore, the information provided is statistically relevant due to the number of molecules probed within the bulk sample and the absence of artefacts induced by the sample preparation steps required by other techniques.”
Access to SANS, however, is heavily overbooked at the ILL. “Only a small fraction of the proposals submitted can be carried out,” explains Schweins. “The ILL is dedicated to the provision of neutrons for society and the immense societal importance of this work was immediately recognised.” The SANS spectra acquired using the ILL’s D11 instrument demonstrated similar molecular packing and composition of hydrogel fibres with and without the addition of zidovudine.
The superior biostability of the hydrogel formed by the peptoid-peptide hybrid was demonstrated by tests carried out in vitro over a period of 28 days. The drug release profile showed steady diffusion over the same period, with approximately 80% of the drug remaining at the end of the study. A dosage interval greater than the initial 28-day objective could therefore be achieved using this peptoid-peptide formulation. Finally, absorption of clinically relevant zidovudine concentrations was demonstrated in vivo for 35 days. The results, recently published in the Journal of the American Chemical Society, provide the first proof-of-concept for a biostable hydrogel depot, formed in response to a physiological trigger from a novel peptoid-peptide material, for the sustained release of a low-molecular-weight drug at clinically relevant concentrations for an extended period.
Clinical translation of this new drug delivery technology requires further safety and efficacy studies in vivo. Work involving neutron techniques is also planned to optimise the material’s formulation as a stable powder that can be readily reconstituted in water prior to injection to ensure widespread provision and distribution. Future perspectives also include extending the improved treatment, prevention and adherence of long-acting drug delivery systems to any condition requiring sustained drug release (e.g. cancer, tuberculosis, malaria) or area of the body in which drugs are difficult to administer (e.g. ocular, spinal).
Supplementary Cover Art published in Journal of the American Chemical Society
ILL Instrument: D11
Reference: Sophie M. Coulter, Sreekanth Pentlavalli, Yuming An, Lalitkumar K. Vora, Emily R. Cross, Jessica V. Moore, Han Sun, Ralf Schweins, Helen O. McCarthy, Garry Laverty, In Situ Forming, Enzyme-Responsive Peptoid-Peptide Hydrogels: An Advanced Long-Acting Injectable Drug Delivery System, Journal of the American Chemical Society (2024) 0002-7863.
DOI : https://doi.org/10.1021/jacs.4c03751
ILL Contact: Ralph Schweins