Biodegradable poly(ethylene glycol) hydrogels based on a self-elimination degradation mechanism

Abstract

Two vinyl sulfone functionalized crosslinkers were developed for the purpose of preparing degradable poly(ethylene glycol) (PEG) hydrogels (EMXL and GABA-EMXL hydrogels). A self-elimination degradation mechanism in which an N-terminal residue of a glutamine is converted to pyroglutamic acid with subsequent release of diamino PEG (DAP) is proposed. The hydrogels were formed via Michael addition by mixing degradable or nondegradable crosslinkers and copolymer {4% w/v; poly[PEG-alt-poly(mercapto-succinic acid)]} at room temperature in phosphate buffer (PB, pH = 7.4). Hydrogel degradation was characterized by assessing diamino PEG release and examining morphological changes as well as the swelling and weight loss ratio under physiological conditions (37 degrees C). Degradation of EMXL and GABA-EMXL hydrogels occurred by surface erosion (confirmed by SEM). GABA-EMXL degradation was significantly faster (approximately 3-fold) than EMXL; however, the degradation of both hydrogels in mouse plasma was 12-times slower than in PBS. The slower degradation rate in plasma as compared to buffer is consistent with the presence of gamma-glutamyltransferase, gamma-glutamylcyclotransferase and/or glutaminyl cyclase (QC), which have been shown to suppress pyroglutamic acid formation. The current studies suggest that EMXL and GABA-EMXL hydrogels may have biomedical applications where 1-2 week degradation timeframes are optimal.

Fig. 1

Representation of 5a and 5b degradable crosslinkers.

Fig. 2

MALDI-TOF-MS of 5a and its intermediates (2a–5a). Signals are marked corresponding to peaks for 2a (A; calculated: 4350, observed: 4502), 3a (B; calculated: 4174, observed: 4163), 4a (C; calculated: 4704, observed: 4633), and 5a (D; calculated: 4374, observed: 4206).

Fig. 3

MALDI-TOF-MS of 5b and its intermediates (2b–5b). Signals are marked corresponding to peaks for 2b (A; calculated: 4516, observed: 4614), 3b (B; calculated: 4340, observed: 4499), 4b (C; calculated: 4870, observed: 4737), and 5b (D; calculated: 4540, observed: 4558).

Fig. 4

Scanning electron micrographs (SEM) of EMXL (A and B) and GABA-EMXL (C and D) hydrogels. The hydrogels were prepared in PB (20 mM, pH=7.4)

Fig. 5

Cumulative release of FITC-Dextran (20 kDa) from degradable (EMXL and GABA-EMXL) and nondegradable hydrogels (NDH) at 37 °C in (A) PBS (pH 7.4) and (B) Mouse Plasma (mean ± S.D., n=3).

Fig. 6

Swelling ratios % (W_t/W₀×100) profile of EMXL and GABA-EMXL hydrogels at 37 °C in (A) PBS (pH 7.4) and (B) Mouse Plasma (mean ± S.D. n=3).

Fig. 7

Degradation of EMXL and GABA-EMXL hydrogels at 37 °C in (A) PBS (pH 7.4); and (B) mouse plasma. Hydrogel degradation behaviors were measured using fluorescamine assay (mean ± S.D., n=3).

Fig. 8

Scanning electron micrograph (SEM) images of the GABA-EMXL hydrogels after incubation in PBS at 37 °C. Samples were removed from incubated hydrogel at regular time intervals (0, 3, 5, and 8 h) and freeze dried. (A) 0 h. (B) 3 h. (C) 5 h. (D) 8 h.

Fig. 9

Degradation profile of GABA-EMXL hydrogel. Studies were done at 37 °C in PBS (pH=7.4).

Fig. 10

Degradation mechanism of EMXL (A; via five membered cyclic intermediate) and GABA-EMXL (B; via ten membered cyclic intermediate) hydrogels.

Fig. 11

MALDI-TOF-MS spectrum of the degraded hydrogel. The peak for DAP was detected at 3582.76 Da.

Fig. 12

MALDI-TOF-MS of the degraded GABA-EMXL hydrogel. The spectrum shows that there is no signal for GABA (m/z= 103) or cyclic GABA (m/z= 83).

Scheme 1

Synthesis of 5a and 5b crosslinkers, a) DAP (3340 Da), PyBOP, DIEA, DMF, RT, 8 h; b) DTT, DMF, Na2CO3, RT, 18 h; c) HBVS, DIEA, DMF, RT, 8 h; d) 3% hydrazine in DMF, RT, 3 h.