Novel dexamethasone-HPMA copolymer conjugate and its potential application in treatment of rheumatoid arthritis

Abstract

Rheumatoid arthritis (RA) is a chronic autoimmune disease of unknown etiology. Effective treatment of this disorder has been hampered by the lack of availability of agents that selectively target affected joint tissue. We developed a novel pH-sensitive drug delivery system of dexamethasone (Dex) based on an N-(2-hydroxypropyl)methacrylamide copolymer (P-Dex) and have shown that the delivery system specifically accumulates in inflamed joints in an animal model of arthritis. We hypothesize that the arthrotropism of the delivery system and the local acidosis-mediated drug release provide superior therapeutic efficacy and potentially reduced side effects in RA treatment. The initial in vitro drug-release study confirmed that the Dex release is indeed dependent upon the environmental pH. At pH 5, 37 degrees C, the conjugate shows the highest level of drug release. When administered systemically in an adjuvant-induced arthritis rat model, P-Dex offers superior and longer-lasting anti-inflammatory effects compared with systemically administered free Dex. In addition, greater bone and cartilage preservation was observed with the P-Dex treatment compared with free Dex treatment. Our data indicate that the differential effect of the conjugate is related to its selective accumulation, potential macrophage-mediated retention, and pH-sensitive drug release (extracellular and intracellular) in arthritic joints. This newly developed drug delivery system provides a unique method for selective targeting of glucocorticoids to inflamed joints which may potentially reduce systemic side effects.

Figure 1

The synthesis of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-dexamethasone conjugate. Conjugation to dexamethasone may occur at either the 3 or the 20 carbonyl group (an example of the latter is shown). AIBN, 2,2'-azobisisobutyronitrile; Boc-NHNH₂, carbazic acid tert-butyl ester; DCC, N, N'-dicyclohexylcarbodiimide; TFA, trifluroacetic acid.

Figure 2

In vitro dexamethasone (Dex) release from N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-Dex conjugate at different temperatures and pH levels. n = 3, standard deviation is less than 5% of mean value.

Figure 3

Bone mineral density (BMD) measurement of healthy and adjuvant-induced arthritis (AIA) rats. BMD was evaluated with peripheral dual x-ray absorptiometry at the ankle, femur, and the fourth and fifth lumbar vertebral bodies. *Significantly different from the healthy control group, p < 0.05. **Significantly different from the AIA+saline group, p < 0.05. ***Significantly different from the AIA+Dex group, p < 0.05. Dex, dexamethasone; P-Dex, N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-dexamethasone conjugate.

Figure 4

Cancellous bone osteoclast surface measurement in the calcaneus of healthy and adjuvant-induced arthritis (AIA) rats. Significant differences were observed between the following groups: AIA+P-Dex versus AIA+Dex, AIA+P-Dex versus AIA+saline, AIA+Dex versus AIA+saline, AIA+Dex versus healthy, and AIA+saline versus healthy. Differences observed between the AIA+P-Dex versus the healthy group were not significant. Dex, dexamethasone; P-Dex, N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-dexamethasone conjugate.

Figure 5

Histological features of the ankle joint from adjuvant-induced arthritis (AIA) rats. (a) Tibia (Tib) astrogalus (Ast) joint from rat with saline injection illustrating extensive bone loss, inflammation, and cartilage erosion (arrows). (b) Same region from a P-Dex-treated animal showing intact articular cartilage with less subchondral bone erosion. (c) Higher-power photomicrograph of cancellous bone from a Dex-treated rat showing extensive osteoclastic (arrows) bone resorption. (d) Same region from a P-Dex-treated rat showing much less eroded bone compared to the Dex-treated rat (c). Dex, dexamethasone; P-Dex, N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-dexamethasone conjugate.