Noninvasive in vivo percutaneous absorption measurements using X-ray fluorescence

Monitoring the in vivo delivery of proteins from carbomer hydrogels by X-ray fluorescence

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 150 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.

Monitoring the retention of a protein antigen in complete Freund's adjuvant, alum, and pluronic F-127 gel formulations by X-ray fluorescence

Adjuvants function by protecting antigens from rapid degradation or dispersal. The effectiveness of experimental adjuvants can be assessed by measuring antibody titers to the antigen of interest or, less frequently, by evaluating the retention and distribution of antigen at the application site. In this study, we used X-ray fluorescence (XRF) to monitor the release of an iodinated protein (I-bovine serum albumin) from several adjuvant formulations after its subcutaneous injection in rats. The interaction of the tagged antigen with an external Am-241 source leads to the emission of iodine X-rays from the application site; the number of these X-rays is proportional to the concentration of the protein remaining at the injection site. The disappearance of the iodine X-rays, and hence the antigen, from the injection site followed first-order kinetics for all adjuvant formulations tested; mean half-life values were as follows: in 50% Freund's adjuvant, 17.1 +/- 1.1 h; in 4-hour-old 25% Alum, 11.78 +/- 0.08 h; in 4-h-old 50% Alum, 13.2 +/- 2 h; in 3-day-old 50% Alum, 15.8 +/- 1.5 h; and in 240 mg/mL Pluronic F-127, 7.9 +/- 0.7 h. We conclude that XRF is an easy, reliable, noninvasive method to monitor the retention of antigens in these adjuvant solutions.