Injectable intratumoral depot of thermally responsive polypeptide-radionuclide conjugates delays tumor progression in a mouse model

Abstract

This study evaluated a biodegradable drug delivery system for local cancer radiotherapy consisting of a thermally sensitive elastin-like polypeptide (ELP) conjugated to a therapeutic radionuclide. Two ELPs (49 kDa) were synthesized using genetic engineering to test the hypothesis that injectable biopolymeric depots can retain radionuclides locally and reduce the growth of tumors. A thermally sensitive polypeptide, ELP(1), was designed to spontaneously undergo a soluble-insoluble phase transition (forming viscous microparticles) between room temperature and body temperature upon intratumoral injection, while ELP(2) was designed to remain soluble upon injection and to serve as a negative control for the effect of aggregate assembly. After intratumoral administration of radionuclide conjugates of ELPs into implanted tumor xenografts in nude mice, their retention within the tumor, spatio-temporal distribution, and therapeutic effect were quantified. The residence time of the radionuclide-ELP(1) in the tumor was significantly longer than the thermally insensitive ELP(2) conjugate. In addition, the thermal transition of ELP(1) significantly protected the conjugated radionuclide from dehalogenation, whereas the conjugated radionuclide on ELP(2) was quickly eliminated from the tumor and cleaved from the biopolymer. These attributes of the thermally sensitive ELP(1) depot improved the antitumor efficacy of iodine-131 compared to the soluble ELP(2) control. This novel injectable and biodegradable depot has the potential to control advanced-stage cancers by reducing the bulk of inoperable tumors, enabling surgical removal of de-bulked tumors, and preserving healthy tissues.

Figure 1. In vitro thermal behavior and degradation of ELPs

A, ELP₁ has a T_t below body temperature over a large range of concentrations. ELP₂ has a high T_t and over the identical concentration range, and is not expected to transition at body temperature. B, The percentage of iodine released was determined over a period of one week at 37 °C. In both PBS and mouse plasma, the ELP₁ (insoluble) conjugates released only a small fraction of the iodine compared to that liberated from ELP₂ (soluble) conjugates. The data are expressed as mean ± SEM (n = 4–5). * indicates significant difference (p < 0.01)

Figure 2. Tumor retention of ELP and radioactive ELPs after intratumoral infusion

A, tumor retention of ELP after intratumoral infusion of [¹⁴C]ELPs in mice bearing FaDu tumor. The ID%/tumor of 0, 24, 48 and 72 hr after administration are expressed as mean (n = 8–10); error bars, SEM; * indicates significant difference (p < 0.01; t-test); B, tumor retention of radioactivity loaded on ELPs after intratumoral infusion of [¹²⁵I]ELPs in mice bearing 4T1 tumor. The ID%/tumor at 0, 24, 48, 72 hr and 1 week after administration are expressed as mean (n = 9); error bars, SEM; * indicates significant difference (p< 0.01; t-test). NOTE: Fig 2A shows retention of ELP after injection, whereas Fig 2B directly assays for radioiodine retention in the tumor. C, ELP-AF488 fluorescence imaging following intratumoral administration. Epifluorescent images of tumor sections at these time points verify the increase in tumor retention time (AF488 = green, Hoechst = blue, scale bar = 100 μm)).

Figure 3. Dose-dependent response of tumors to ¹³¹I -loaded ELPs

A, Relative tumor volume (normalized to initial tumor volume) for mice infused with different doses of radioactivity, which are expressed as mean (n = 6); error bars, SEM; * indicates statistically significant difference (p < 0.05, t-test) from unlabelled ELP control. B. Mean body weights of 4T1 xenograft-bearing nude mice intratumorally injected with unlabeled ELP₁ and increasing doses of [¹³¹I]ELP₁. Data represent mean (n = 6); error bars, SEM.

Figure 4. Antitumor efficacy of ¹³¹I -loaded ELPs

A, Comparison of ¹³¹I -loaded ELP₁ and ELP₂. Data represent mean (n = 9); error bars, SEM; * indicates P < 0.05 vs. ELP₂ or saline. B, The mouse survival was analyzed by the Kaplan-Meier method and compared by Mann-Whitney Test. * indicates P < 0.01 Vs. ELP₁ C, Mean body weights of 4T1 xenograft-bearing nude mice given intratumoral injections of saline or [¹³¹I]ELP₁. Data represent mean (n = 9); error bars, SEM.