David R. Friend

Colon-specific drug delivery

Abstract The delivery of drugs to the colon has a number of important implications in the field of pharmacotherapy. Several diseases, including IBD, can be treated more effectively by local delivery of antiinflammatory agents to the large intestine. SASP is the classic prodrug used to deliver 5-ASA for localized chemotherapy of IBD. Due to adverse side-effects of SASP, a number of newer 5-ASA prodrugs are under development. The use of azo-reductase activity of colonic microflora has also been used to liberate 5-ASA from polymeric prodrugs and more recently to catalyze the partial degradation of hydrogels in the colon. These hydrogels may be useful in the delivery of peptides and proteins to the colon. The colonic bacteria also produce a wide array of glycosidases capable of hydrolyzing simple glycosides as well as polysaccharides. These enzymes have been exploited to release drugs in the large intestine. A number of studies have indicated that corticosteroids released from glycoside prodrugs are absorbed from the lumen of the large intestine and lead to much higher colonic tissue levels than are possible when the parent corticosteroid is administered systemically. The colon is also a potential site for absorption of peptides and proteins. Some potential limitations involved in oral peptide and protein delivery from the colon are discussed. In addition, the use of specific receptors and lectins on colonic mucosal cells offer the potential to deliver drugs to the colonic mucosa in a selective way. Interestingly, certain lectins have significantly increased binding to colonic mucin in several disease states: IBD and large bowel carcinoma. Exploiting these differences could lead to new, biologically oriented colonic drug delivery systems that deliver drugs selectively to the target cells, thereby reducing systemic exposure.

In vitro skin permeation techniques

Abstract This paper reviews various techniques used to assess permeation of drugs into and through skin under in vitro conditions. There have been a wide variety of diffusion cells designed for in vitro measurement of skin permeation. These cells have generally been designed in one of two ways: side-by-side (bichambers) and vertical in vivo mimic diffusion cells. A primary goal of in vitro permeation studies is the prediction of skin permeation in vivo, in this regard, side-by-side diffusional cells are useful in delineating mechanisms of permeation under controlled conditions, but are of more limited usefulness in predicting skin permeation in vivo. Vertical cells are more versatile because a wide variety of experimental conditions can be used to gain information useful in the evaluation of formulations ultimately destined for clinical use. A number of diffusion cell designs are reviewed and some of their advantages and disadvantages are discussed. In addition to diffusion cells used to assess passive diffusion, iontophoretic- and phonophoretic-enhanced skin permeation in vitro is also considered. Until the use of differentiated keratinocytes in culture becomes inexpensive and the relationships between skin permeation in vitro and in vivo are established, skin permeability will be measured using excised animal or human skin in diffusion cells.

Synthesis and chemical stability of glucocorticoid-dextran esters: potential prodrugs for colon-specific delivery☆

Dextran ester prodrugs have been proposed as a means of delivering drug to the colon. In this study, methylprednisolone and dexamethasone were covalently attached to dextran (Mw = 72 600) by the use of a succinate linker. In addition, dexamethasone was attached by glutaric acid to investigate the effect of linker molecule on hydrolysis kinetics. The kinetics of degradation of the hemiesters and corresponding dextran conjugates were measured as a function of pH and temperature. Intramolecular migration of the linker molecule from the 21- to the 17-position on the glucocorticoid occurred in all three hemiesters, although to a greater extent in methylprednisolone-hemisuccinate. The dextran conjugates were also incubated at 37°C, pH 6.8 and the chemical degradation half-lives were as follows: dexamethasone-succinate-dextran 75 h; dexamethasone-glutarate-dextran 103 h and methylprednisolone-succinate-dextran 82 h. Incubation of dexamethasone-21-hemisuccinate with rat gastrointestinal (GI) tract luminal contents indicated that the hemiester is hydrolyzed throughout the GI tract. Greatest esterase activity, however, was found in the small intestine. By contrast, little drug (< 3%) was released from dexamethasone-succinate-dextran during incubation with small intestinal contents despite the high esterase activity. Dexamethasone and dexamethasone-21-hemisuccinate were released at faster rates during incubation with cecum and colon contents. This combination of chemical stability and selective enzyme-mediated drug release in the large intestine indicates that these dextran prodrugs have potential in colon-specific delivery of glucocorticoids.

Transdermal delivery of levonorgestrel I: Alkanols as permeation enhancers in vitro

The effect of alcoholic penetration enhancers on the transdermal delivery of levonorgestrel (LN) was investigated. A series of albanais was tested in vitro it for their ability to enhance the delivery of LN through excised rat skins. The steady-state flux of LN was measured using donor phase compositions of water fethanol mixtures, and the neat straight chain alkanols ethanol, propanol, butanol, pentanol, hexanol, and octanol. The steady-state flux of LN was found to increase as the alkyl chain length increased from C2 to C4. This was followed by a decrease in the steady-state flux as the alkyi chain length increased above 1 -butanol. Three secondary alkanols, 2-propanol, 2-butanol, and 2-pentanol were also tested as penetration enhancers. The same trend was evident: steady-state flux was highest for 2-butanol with 2-propanol and 2-pentanol giving lower steady-state fluxes. The flux of LN was lower for secondary alkanols relative to the corresponding primary alkanol. Propylene glycol was also tested as an enhancer. Overall, the steady-state flux using this penetration enhancer was about that of water, which was low relative to all the alkanols. Flux of LN through human cadaver skin from pure ethanol was about four times lower than through rat skin. A mechanism for permeability enhancement based on the alkanol structure and water-solubility is presented as well as a discussion of the data as it relates to development of a transdermal delivery system for LN.

Transdermal delivery of levonorgestrel. VIII. Effect of enhancers on rat skin, hairless mouse skin, hairless guinea pig skin, and human skin

Abstract The penetration enhancing effect of ethyl acetate, with or without ethanol as a cosolvent, was evaluated in vitro on rat skin, hairless mouse skin, hairless guinea pig skin, and human cadaver skin using the contraceptive drug levonorgestrel. Under the influence of the enhancers, the relative skin permeability of levonorgestrel through the four skin types showed a clear trend: hairless mouse skin >hairless guinea pig skin ∼ rat skin > human skin. These results show that rodent skins are poor models for human skin under the conditions used. The steady-state flux of levonorgestrel through human skin was increased about 7-fold 0.03 μg/cm 2 h vs 0.2 μg/cm 2 h when neat ethyl acetate was used in place of ethanol as the donor vehicle. Adding polyethylene glycol 400 (PEG 400; 40% v/v in saline) to the receptor solution increased the steady-state flux of levonorgestrel through human skin only slightly relative to saline as a receptor solution. The lag time of levonorgestrel through human skin was longer relative to that for the rodent skins tested. The flux of ethyl acetate and ethanol through all the skins was also measured. The rodent skins were all much more permeable towards both solvents than was human skin. For each skin type tested, there appeared to be a direct correlation between the cumulative amount of solvent permeating through the skin and the cumulative amount of drug permeating through the skin.

In Vitro Evaluation of Dexamethasone-β-D-Glucuronide for Colon-Specific Drug Delivery

Dexamethasone-β-D-glucuronide is a potential prodrug for colonic delivery of the antiinflammatory corticosteroid dexamethasone. Previous studies [T. R. Tozer et al., Pharm. Res. 8:445–454 (1991)] indicated that a glucoside prodrug of dexamethasone was susceptible to hydrolysis in the upper gastrointestinal tract. Resistance of dexamethasone-β-D-glucuronide to hydrolysis in the upper gastrointestinal tract was therefore assessed. Conventional, germfree, and colitic rats were used to examine enzyme levels along the gastrointestinal tract to compare the stability of two model substrates (p-nitrophenyl-β-D-glucoside and -β-D-glucuronide) and to evaluate the prodrug dexamethasone-β-D-glucuronide. Hydrolytic activity was examined in the luminal contents, mucosa, and underlying muscle/connective tissues in all three types of rats. Enzymatic activity (β-D-glucosidase and β-D-glucuronidase) was greatest in the lumen of cecum and colon of conventional rats. In contrast, germ-free rats exhibited relatively high levels of β-D-glucosidase activity (about 80% of total activity in the conventional rats) in the proximal small intestine (PSI) and the distal small intestine (DSI). Rats with induced colitis (acetic acid) showed reduced levels of luminal β-D-glucuronidase activity in the large intestine; however, β-D-glucosidase activity was relatively unchanged relative to that of the conventional rat. Mucosal β-D-glucuronidase activity was significantly lower in the colitic rats compared with that in the conventional animals. Despite reduced luminal levels of β-D-glucuronidase activity in the colitic rats, there was still a sharp gradient of activity between the small and the large intestines. Permeability of the glucoside and glucuronide prodrugs of dexamethasone through a monolayer of Caco-2 cells was relatively low compared to that of dexamethasone. The results indicate that dexamethasone-β-D-glucuronide should be relatively stable and poorly absorbed in the upper gastrointestinal tract. Once the compound reaches the large intestine, it should be hydrolyzed to dexamethasone and glucuronic acid. Specificity of colonic delivery in humans should be even greater due to lower levels of β-D-glucuronidase activity in the small intestine compared with that in the laboratory rat.

A glucocorticoid prodrug facilitates normal mucosal function in rat colitis without adrenal suppression

BACKGROUND/AIMS Glucocorticoids remain the foundation of therapy for acute ulcerative colitis despite systemic side effects that limit their use. Prodrugs that selectively deliver glucocorticoids to the colon may lower the required dose and side effects. The aim of this study was to assess the efficacy of a newly synthesized glucocorticoid-dextran prodrug. METHODS Novel glucocorticoid-dextran prodrug conjugates in which dexamethasone and methylprednisolone were attached to dextran were synthesized using the dicarboxylic acid linkers, succinate and glutarate. The efficacy of the dextran prodrug conjugates and their free glucocorticoids was tested in an acetic acid-induced model of colitis. Repair of the colitis and mucosal function was assessed by measuring net intestinal fluid absorption, macroscopic ulceration, and myeloperoxidase activity. Glucocorticoid toxicity was evaluated by measuring plasma adrenocorticotropic hormone and serum corticosterone levels. RESULTS The prodrug dexamethasone-succinate-dextran was nine times more potent and dexamethasone-glutarate-dextran three times more potent than free dexamethasone in accelerating mucosal repair. Similarly, methylprednisolone-succinate-dextran was four times more potent than free methylprednisolone. The dextran prodrug conjugates affected adrenocorticotropic hormone and corticosterone levels only at the highest doses in contrast to free dexamethasone and methylprednisolone, which caused marked adrenal suppression at all doses. CONCLUSIONS The results show that recently synthesized glucocorticoid-dextran prodrug conjugates can be administered orally to facilitate mucosal repair in rat colitis without adrenosuppression.

Colon-specific delivery of dexamethasone from a glucoside prodrug in the guinea pig

Dexamethasone-β-D-glucoside is a potential prodrug for colonic delivery of the antiinflammatory agent, dexamethasone. The ability of this prodrug to deliver dexamethasone selectively to the colon depends not only on its being slowly absorbed from the alimentary canal, but also on its having chemical and enzymatic stability in the stomach and small intestine. Once reaching the large bowel, it should be quantitatively hydrolyzed to release the active agent. The potential of dexamethasone-β-D-glucoside for colon-specific delivery of dexamethasone is assessed by determining the rates of its hydrolysis down the alimentary canal of the guinea pig, an animal in which an inflammatory bowel disease model has been developed. The hydrolytic activity is examined in tissues and luminal contents of the stomach, proximal and distal segments of the small intestine, cecum, and colon. For the tissues, the greatest hydrolytic activity is in the proximal small intestine, while the stomach, cecum, and colon have only moderate activity. In contrast, the contents of the cecum and colon show greater activity than the contents of the small intestine and stomach. The luminal contents retained β-glucosidase activity even after repeated centrifugation and resuspension in a buffer. The activity was unaffected by homogenization. These observations suggest that hydrolytic activity is associated with enzymes located on the surface of luminal cells. The movement and hydrolysis of dexamethasone-β-D-glucoside down the gastrointestinal tract of the guinea pig are also examined. About 20 to 30% of an oral dose appears to reach the cecum. Here the prodrug is rapidly hydrolyzed to the active drug. From intravenous administration of the prodrug and drug, it is apparent that dexamethasone-β-D-glucoside is poorly absorbed in the gastrointestinal tract (bioavailability, <1%). There is a ninefold selective advantage for delivery of dexamethasone in cecal tissues in the guinea pig under the conditions of this experiment. Thus, there is a potential for a decrease in the usual dose and a concomitant reduction in the systemic exposure to dexamethasone. Because humans have much less glucosidase activity in the small intestine, even greater site-selective delivery to the cecum and colon is expected.

Simple alkyl esters as skin permeation enhancers

Abstract The use of simple alkyl esters, in particular ethyl acetate, as skin permeation enhancers for drugs is presented. Ethyl acetate was found to increase the transdermal flux of levonorgestrel, estradiol, hydrocortisone, 5-fluorouracil, and nifedipine through rat skin in vitro relative to that when ethanol or water were used as reference solvents. Relative to water, the flux of levonorgestrel was increased 80 fold using pure ethyl acetate. Relative to pure ethanol as a reference permeation enhancer, the flux of levonorgestrel was increased 12 to 15 fold using either pure ethyl acetate or volume fractions of ethyl acetate in ethanol down to 0.18 as permeation enhancers. Butyl acetate was also tested with levonorgestrel and was found to increase the flux about 7 fold relative to pure ethanol. The fluxes of estradiol, hydrocortisone, 5-fluorouracil, and nifedipine were increased from 75 to 650 fold, depending on the drug, when ethyl acetate was used as a permeation enhancer relative to that when water was used as an enhancer, and from 9 to 34 fold relative to that when ethanol was used as an enhancer. Also discussed are the possible mechanisms of action by which ethyl acetate increases the percutaneous absorption of drugs and use of ethyl acetate in transdermal drug delivery systems.

In vivo pharmacokinetic study for the assessment of poly(L-aspartic acid) as a drug carrier for colon-specific drug delivery

Glucocorticoids remain one of the mainstays of therapy for acute attacks of inflammatory bowel disease despite systemic side effects that limit their use. Prodrugs that selectively deliver glucocorticoids to the colon may lower the required dose and side effects. Because enzymes of gut microflora are able to cleave certain peptide and ester bonds, the ability of an ester prodrug consisting of dexamethasone (DX) as model drug and poly(L-aspartic acid) (weight-average mol wt=30,000) as drug carrier was investigated to selectively release the drug in the large intestine. Prodrug and drug solutions (1.18 mg DX/ml DMSO) were administered to two groups of male Sprague-Dawley rats by intragastric infusion using an ALZET® osmotic pump. All rats were infused for sufficient time to achieve steady state in both blood and GI-tract tissues. DX concentrations in blood and tissue samples were measured with HPLC. The steady state DX concentrations at these sites were used to calculate a drug delivery index (DDI). DX blood concentrations were significantly lower (p<0.05) after intragastric administration of the prodrug. Moreover, prodrug administration resulted in significantly higher DX concentrations in the cecum and colon mucosa and the cecum muscle tissue compared to DX administration (p<0.05). The prodrug led to an increase of the DX concentration in the large intestinal tissues by factors of 1.3–2.0 and to an 1.3-fold decrease of DX blood concentrations. Thus, this novel conjugate should both increase efficacy and reduce toxicity to some extent.