An injectable and in situ-gelling biopolymer for sustained drug release following perineural administration

Abstract

Study design: This study evaluated whether the aggregation behavior of a thermally responsive elastin-like polypeptide (ELP) prolongs protein residence time at the dorsal root ganglion (DRG). This work involves development of a sustained-release drug delivery vehicle to provide high and sustained levels of biologic therapeutics to the dorsal root ganglion while minimizing systemic exposure.

Objective: To study the potential of the ELP biopolymer to sustain release and lower systemic exposure of bioactive peptides following perineural administration.

Summary of background data: Anticytokine treatment for lumbar radiculopathy may offer clinical improvement, but exposes patients to systemic toxicities of immunosuppression. ELPs are environmentally responsive polypeptides that undergo a phase transition on heating to form an insoluble aggregate. Drug conjugates with ELP exhibit both temperature-sensitivity and in vitro bioactivity. Monomer resolubilization yields solution-phase molecules, and this reversible aggregation behavior may create a perineural drug depot to sustain drug delivery to an inflamed nerve.

Methods: This experiment involved 48 rats in which radiolabeled ELPs (aggregating or soluble) were injected overlying the L5 dorsal root ganglion. Animals were killed at 6 different time points, and radioactivity associated with the injected segment, serum, and other tissues was evaluated.

Results: The aggregating ELP demonstrated a 7-fold longer perineural half-life compared with the soluble ELP. This supports the hypothesis that the aggregating ELP forms a depot from which slow resolubilization and clearance provides sustained, local protein release. Furthermore, serum radioactivity reached a lower peak for the aggregating group, demonstrating slower absorption of the aggregating protein into the systemic circulation.

Conclusion: These results suggest that ELP aggregation confer the benefit of perineural compartment longevity for bioactive therapeutics delivered fused with this carrier. This may sustain release of potent immunomodulator therapeutics to treat local neuroinflammation. Desirable features include delivery of high local doses and protection against systemic exposure and associated toxicity.

Figure 1

Incubation of both soluble and aggregating ELP sequences in PBS or rat serum at 37°C. Significant effects (ANOVA) were detected for both tested factors: ELP sequence (P < 0.01) and incubation conditions (P < 0.01). Serum degradation exceeded baseline from day 1 onwards for the soluble ELP (*, Dunn’s tests, α = 0.05) and from day 7 onwards (*, Dunn’s tests, α = 0.05). Greater fraction of degradation products were measured for the soluble ELP at nearly all time points (#, Dunn’s tests, α = 0.05).

Figure 2

Clearance of ELP from the perineural delivery to the right L5 DRG is well described by a first-order exponential model for both soluble (upper, r² = 0.97) and aggregating (lower, r² = 0.94) sequences. Data are expressed as mean ± SD (n = 4) and presented with first-order exponential decay fits and 95% confidence intervals. Half-lives are 5.5 ± 0.5 hours for the soluble ELP and 39 ± 5 hours for the aggregating ELP (Student t test, P < 0.01).

Figure 3

Serum fraction of soluble and aggregating ELP, normalized to the injected dose. Data are expressed as mean ± SD (n = 4) with the maximum serum fraction of both ELPs observed at 24 hours. Maximum magnitudes were 23 ± 5% ID for the soluble ELP and 1.6 = 0.3% ID for the aggregating ELP.

Figure 4

Profile of soluble (upper) and aggregating (lower) ELP distribution to peripheral tissues, normalized to the injected dose. Data are expressed as mean ± SD (n = 4) Accumulation of radioactivity was noted in the kidney for soluble ELP (significantly higher at 24 hours than all other times, Dunn’s tests, α = 0.05) and for aggregating ELP (significantly higher at 96 hours than 0, 6, and 336 hours, Dunn’s tests, α = 0.05). The soluble ELP did not accumulate in the brain, liver, or muscle (ANOVA, P > 0.05); but did peak at the C2 motion segment at 24 hours (significantly higher than at 0 and 48 hours, Dunn’s test, α = 0.05). The aggregating ELP did not accumulate in the brain, liver, C2 motion segment, or muscle (ANOVA, P > 0.05).