Introduction: Predictive in vitro models are vital to reducing the cost and time associated with the progression to clinical use of drug delivery systems. This has been especially true in the translation of phase-sensitive in situ forming implants (ISFIs), which have been extensively studied as a means of achieving and sustaining therapeutic concentrations of chemotherapeutic drug at the site of a malignant tumors. The lack of correlation between in vitro and in vivo release profiles has hindered these systems from being used in clinical applications. While in the past, only polymer concentration, solvent type, and bath side components have been shown to alter the release of drug in vivo, recent studies from our lab have shown that the stiffness of injection site can also have a direct effect on release properties[1]. We have hypothesized that the osmotically-driven swelling of these polymer implants is inhibited by the reactive forces of a surrounding tissue, resulting in an increased drug release. To investigate the effect of injection site stiffness on drug release, we have developed hydrogel phantoms with different elastic moduli, capable of inhibiting implant swelling.
Materials and Methods: Polyacrylamide phantoms of different moduli were made with varied ratios of acrylamide to water, crosslinked with bis-acrylamide inside a mold. The elastic moduli of phantoms was determined using standard unconfined compression testing. ISF implant solutions were made by dissolving a 50:50 17 kDa poly(lactic-co-glycolic acid) (PLGA) polymer in 1-methyl-2-pyrrilidinone (NMP) with a 1% mass ratio of fluorescein, used as a mock drug. Implants were formed by either direct injection into a solution of PBS or into polyacrylamide phantoms, which were also placed in a bath of PBS. Implants were kept at 37oC in an incubator shaker for predetermined time points. To measure swelling, the injected polymer mass was measured initially upon injection and at each time point. The polymer implants were scanned diagnostic ultrasound equipped with a 12 MHz transducer. To measure drug release, implants were harvested and degraded in 2M NaOH solution. Residual drug was measured using a fluorescence plate reader.
Results and Discussion: Swelling of polymer implants was greatest in PBS solution, reflecting the high swelling potential in an unconstrained environment (Figure 1A). The swelling decreased as the stiffness of the phantoms increased and the resulting morphology differences can be observed in the 2D US scans (Figure 1B).
Cumulative fluorescein release at the end of the study for 20 kPa, 10 kPa, 1 kPa, and PBS was 98.9%, 88.5%, 79.1% and 46.7%, respectively (Figure 2). This is due to the constraining environment of the implant hindering it from swelling, causing its cargo to release faster. Surprisingly, the release data indicates that the stiffness of the implant injection site did not increase the burst release. We speculate that this is due to the low permeability coefficient of the highly crosslinked gels, which creates a higher accumulation of fluorescein outside of the implant and reduces the rate of fluorescein release.
Conclusion: These results confirm that mechanical properties of in vivo environments have an important effect on drug delivery of ISFIs. In the future work, we will incorporate perfusion into the hydrogel phantoms to better model the environment and determine the effects of perfusion on drug release profiles.
This work was supported by the National Institute of Biomedical Imaging and Bioengineering and the National Cancer Institute of the National Institutes of Health under award numbers R01EB016960, T32-EB007509-05 and F31 CA200373-01.
References:
[1] Patel, Ravi B., et al. "Effect of injection site on in situ implant formation and drug release in vivo." Journal of Controlled Release 147.3 (2010): 350-358.