Monitoring the in Vivo Delivery of Proteins from Carbomer Hydrogels by X-Ray Fluorescence

Abstract

Purpose. To measure the effect of protein size on their disappearance from subcutaneously implanted carbomer hydrogel matrices.

Methods. A series of different molecular weight (MW) proteins were iodinated, incorporated into Carbopol hydrogels, injected subcutaneously into rats, and monitored using X-ray fluorescence (XRF).

Results. A 10 mg/mL minimum concentration of Carbopol-940 was necessary before protein [50 mg/mL iodinated bovine serum albumin (I-BSA)] retention times increased with increasing hydrogel concentration. The decreasing protein signal was not caused by outward protein diffusion or iodoprotein hydrolysis. As the protein MW increased, protein retention times lengthened [e.g., 6.2 h for insulin (5.7 kDa) to 13.3 h for thyroglobulin (669 kDa)]. Protein disappearance was monophasic first-order for some proteins and biphasic first-order for others. The disappearance rate constant ranged from 0.093 ± 0.005 h⁻ _1/2 to 0.187 ± 0.057 h⁻ _1/2, indicating gel erosion rather than protein diffusion as the rate-limiting mechanism. Entrapped I-BSA in Carbopol-1342 NF, pH 7.4, and Carbopol 2001-ETD, pH 7.4, gel matrices yielded different disappearance rates and profiles than Carbopol-940. The overall 50% disappearance rate of I-BSA was greatest for Carbopol-1342 NF (41 ± 8 h), followed by Carbopol-2001 ETD (25 ± 2 h) and Carbopol-940 (10.5 ± 0.7 h).

Conclusion. XRF is a noninvasive technique that can be used to follow the status of macromolecules in vivo.