Waleed S.W. Shalaby

In vitro and in vivo studies of enzyme-digestible hydrogels for oral drug delivery

Abstract Gastric retention of enzyme-digestible hydrogels in dogs was studied to develop a platform for longterm oral drug delivery. The movements of hydrogels in the canine stomach and the gastric tissue-hydrogel interactions were visualized in real-time using three imaging techniques: radiography, fluoroscopy, and 2-D ultrasonography. Hydrogels with appropriate size and integrity were retained in the stomach for more than 24 h even under fasted conditions. Gastric retention was prolonged up to 60 h when the 24 h-fasted state was followed by a once-a-day pelletized meal. In vitro drug release from enzyme-digestible hydrogels was examined. The hydrogels were loaded with flavin mononucleotide (FMN) and air-dried for 7 days. FMN was released from the gels in the simulated gastric juice for up to 454 h and 650 h in the presence and absence of pepsin, respectively. About 90% of the loaded FMN was released in 300 h. During this time, the presence of pepsin in the solution did not make any significant difference in the release profiles. When the FMN-containing hydrogels were administered to dogs, the blood concentration of FMN was maintained at the level of 0.75 μg/ml for 24 h under fasted conditions. In each experiment, the presence of a hydrogel in the stomach was confirmed using the three imaging techniques. When the 24-h-fasted state was followed by a once-a-day meal, appreciable FMN concentration was detected in the blood for up to 54 h. This study showed that once-a-day oral drug delivery is possible even with a drug which is absorbed only from the upper small intestine and has a short half-life.

Biochemical and Mechanical Characterization of Enzyme-Digestible Hydrogels

Albumin-cross-linked hydrogels were prepared by free radical polymerization using 1-vinyl-2-pyrrolidinone as a monomer and functionalized albumin as a crosslinking agent. The degree of chemical cross-linking was controlled by varying the degree of albumin functionality and the concentration of albumin. With emphasis placed on the potential use of these hydrogels for long-term oral drug delivery, gel characterization studies examined both the swelling and the mechanical properties in the absence and presence of pepsin. In the absence of pepsin, the equilibrium swelling ratio in simulated gastric fluid ranged from 17 to 55, depending on the degree of albumin functionality and the albumin concentration. Swelling was pH dependent at pHs greater than 7. The uptake of solvent into the dried hydrogels was determined to be Fickian. The integrity of swelling gels was dependent on the concentration of the functionalized albumin as well as on the degree of albumin functionality. In the presence of pepsin, a predominance of either surface or bulk degradation was observed, depending on the functionality of the albumin used as a cross-linker. Gel integrity during pepsin degradation also showed a marked dependence on the albumin functionality.

Use of ultrasound imaging and fluoroscopic imaging to study gastric retention of enzyme-digestible hydrogels

Ultrasound and fluoroscopic imaging techniques were used to monitor the gastric retention of enzyme-digestible hydrogels in the canine stomach. When water was present in the stomach, ultrasound imaging was very effective in monitoring the position of the hydrogel in the stomach, solvent penetration into the gel, and the gastric tissue-gel interactions during peristalsis. Rubbery or fully swollen hydrogels appeared as sonolucent objects with ultrasound imaging. Partially swollen hydrogels displayed a sonolucent outer layer due to solvent penetration and a centrally located bright echo resulting from the acoustic impedance mismatch at the glassy/rubbery interface. The degree of gastric tissue-gel interactions during peristalsis was inversely related to the extent of lumenal distention with water. The effectiveness of peristaltic contractions in driving the hydrogel toward the pyloric sphincter increased as the water was emptied from the stomach. In the absence of water, imaging of the gel with ultrasound became difficult. For this reason, gels were loaded with diatrizoate meglumine/sodium diatrizoate to visualize in real-time using fluoroscopic imaging. Fluoroscopic imaging allowed only indirect assessment of the hydrogel movement during peristalsis and the degree of hydrogel swelling. The gastric retention of the hydrogel under fasted conditions was influenced by the degree of gel deformation in response to peristaltic contractions. Hydrogels with a low degree of deformation during peristalsis showed long gastric retention times. The utilization of ultrasound imaging and fluoroscopic imaging for monitoring dynamic events in the stomach provided information on hydrogel properties which are important to gastric retention. The use of these imaging techniques in the development of long-term oral drug delivery systems is described.

The potential use of poly(methacrylic acid) hydrogels for oral administration of drugs and vaccines to ruminants

Abstract Poly(methacrylic acid) hydrogels were investigated for the delivery of a model antigen to the lower gastrointestinal tract of sheep. Hydrogels were tested by incorporating a radiopaque material, administering them orally to a sheep, and then radiographing the sheep. The potential for loading high molecular weight proteins into hydrogels was determined by absorbing hydrogels with culture supematants of the bacterium Pasteurella haemolytica . The hydrogels were dried, hydrated, and the culture supematants eluted. The eluents were assayed for the presence of the 102 kDa proteinaceous exotoxin. The hydrogels readily bypassed the first stomach and swelled releasing a radiopaque dye into the lower gastrointestinal tract. Chromium-loaded hydrogels were then administered to a sheep and intestinal contents were collected for 5 days. Chromium was detected in the intestinal contents of the sheep for 96 h with peak levels detected at 12–15 h after administration. Eluents of the hydrogels loaded with culture supematants contained readily detectable amounts of the proteinaceous exotoxin. PMA hydrogels were then absorbed with a vaccine consisting of culture supematants of a pulmonary bacterium P. haemolytica . Hydrogels containing vaccine were administered orally to calves. Calves were challenged by an intrabronchial dose of bacteria. The length of time each calf survived was noted. All surviving calves were killed 3 days post-challenge. A post-mortem examination was performed to evaluate the severity of the pneumonic lesions. Vaccinated calves had less pneumonia and lived longer than control calves. Results of this study indicate that poly (methacrylic acid) hydrogels could be used to administer drugs and proteinaceous vaccines orally to ruminants.

Release of dextromethorphan hydrobromide from freeze-dried enzyme-degradable hydrogels

Abstract Dextromethorphan hydrobromide was incorporated into albumin-crosslinked polyvinylpyrrolidone hydrogels by equilibrating the gels in a saturated drug solution followed by freeze drying to entrap the drug in the network. Through freeze drying, highly porous hydrogels containing uniformly dispersed drug were produced. Drug content in the gel increased as a function of the number of drug loading-freeze drying cycles. Solvent penetration into freeze-dried gels resulted in an initial isotropic collapse of the network followed by a gradual increase in gel size due to swelling. In the presence of pepsin, freeze-dried networks degraded at a much faster rate than non-freeze-dried control samples. Drug release from freeze-dried hydrogels was degradation-independent and inconsistent with conventional solute transport mechanisms through swellable low surface area devices. The rate of drug release was dependent on the amount of drug loaded into the freeze-dried matrix. The potential use of these devices for long-term oral drug delivery is discussed.

A Mechanistic Assessment of Enzyme-Induced Degradation of Albumin-Crosslinked Hydrogels

Pepsin-induced degradation of albumin-crosslinked hydrogels was studied as a function of the degree of albumin incorporation in the network and the concentration of pepsin. The degree of albumin incorporation, which represents the sum of chemical crosslinks and physical entanglements in the network, was controlled by changing the concentration of initiator in the monomer solution and the degree of vinylic functionality on albumin. Swelling characterization studies showed that the degree of hydrogel swelling decreased as the concentration of chemical initiator for the polymerization increased or as the degree of vinylic functionality on albumin increased. This indicated that the degree of albumin incorporation in the network increased by raising either the concentration of chemical initiator or the degree of albumin functionality. The rate and mechanism of gel degradation was also dependent on the degree of albumin incorporation in the network. A low degree of albumin incorpora tion resulted in a predominance of surface degradation while a high degree of albumin incorporation resulted in a predominance of bulk degradation. The transition from surface degradation to bulk degradation occurred at lower con centrations of chemical initiator when the degree of vinylic functionality on albumin was high. However, when the degree of vinylic functionality on albumin was low, the transition from surface degradation to bulk degradation was observed at higher concentrations of chemical initiator. The rate of gel degradation became slower as the concentration of pepsin was reduced. The results suggest that the rate and mechanism of hydrogel degradation was de pendent on the steric constraints imposed by polymer chains of the network and on the conformational constraints of the albumin crosslinker.

Synthesis of Enzyme-Digestible, Interpenetrating Hydrogel Networks by Gamma-Irradiation

γ-Irradiation was used to prepare cylindrically shaped, enzyme- digestible hydrogels consisting of a moderately swelling albumin-crosslinked poly(vinyl pyrrolidone) (PVP) hydrogel core which formed a semi-interpene trating network (SIPN) at its outer region with a polyelectrolyte homopolymer of acryloxyethyltrimethylammonium chloride (AETAC). In pepsin-free simu lated gastric fluid (SGF), the presence of a SIPN resulted in only a marginal in crease in swelling as compared to the pure PVP hydrogel. In distilled deionized water, however, the swelling of the 2-phase hydrogel system was dramatically increased. The effects of γ-irradiation on the individual constituent polymers that com prised the 2-phase system were studied. Vinyl pyrrolidone (VP) formed PVP which subsequently crosslinked to form an insoluble three-dimensional net work after 4 h of γ-irradiation. Polymers of AETAC (PAETAC) underwent chain scission following 30 min of γ-irradiation. FA became crosslinked as deter mined by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS- PAGE). The swelling properties of albumin-crosslinked PVP and albumin- crosslinked PAETAC hydrogels were also studied as a function of γ-irradiation time and the concentration of FA in the monomer solution. The equilibrium swelling ratios of the prepared hydrogels in pepsin-free simulated gastric fluid (SGF) were dependent on the γ-irradiation time which affected the crosslinking or chain scission of the polymers. Both the PVP and PAETAC hydrogels under went bulk degradation in pepsin-containing SGF.