Dissociation of insulin oligomers by bile salt micelles and its effect on alpha-chymotrypsin-mediated proteolytic degradation

Bile acid-based advanced drug delivery systems, bilosomes and micelles as novel carriers for therapeutics

Diabetes mellitus affects almost half a billion patients worldwide and results from either destruction of β-cells responsible for insulin secretion or increased tissue resistance to insulin stimulation and the reduction of glycemic control. Novel drug delivery systems can improve treatment efficacy in diabetic patients. The low aqueous solubility of most oral antidiabetic drugs decreases drug bioavailability; therefore, there is a demand for the use of novel methods to overcome this issue. The application of bile acids mixed micelles and bilosomes can provide an enhancement in drug efficacy. Bile acids are amphiphilic steroidal molecules that contain a saturated tetracyclic hydrocarbon cyclopentanoperhydrophenanthrene ring, and consist of three 6-membered rings and a 5-membered ring, a short aliphatic side chain, and a tough steroid nucleus. This review offers a comprehensive and informative data focusing on the great potential of bile acid, their salts, and their derivatives for the development of new antidiabetic drug delivery system.

Improved systemic delivery of insulin by condensed drug loading in a dimpled suppository

The development of peptide therapeutics owing to the advances in biotechnology has overcome some unmet medical needs; however, the route of administration is still limited to injections. Systemic delivery of insulin via an enteral route remains a great challenge due to its instability and low mucosal permeability. In this study, we investigated the effect of drug condensation in a suppository on the efficacy of insulin after rectal administration. Suppositories with dimples are prepared by a mold method using a hard fat (Suppocire^® AM). Insulin or fluorescein isothiocyanate-dextran (molecular weight: 3,000-5,000) (FD4) as a model of a hydrophilic macromolecule was loaded in the dimples, and sealed with other lipids with different melting points. The in vitro release test showed that the time to 50% drug release depends on the melting point of the lipid for sealing but not on the number of dimples. The suppositories with one-, or three-dimple containing insulin and caprylocaproyl macrogol-8 glyceride (Labrasol^®) were administered to rats at 0.5 U/head. The reduction in plasma glucose level was more significant for the one-dimple-type suppository than for the three-dimple-type although the one-dimple-type suppository contained less amount of Labrasol by one-third compared to the three-dimple-type. These results suggest that condensation of an insulin dose in a limited surface area of a suppository improves systemic availability via the rectal route with a reduced amount of an absorption enhancer.

Enhanced insulin absorption from sublingual microemulsions: effect of permeation enhancers

Microemulsions of insulin (50 IU/mL) comprising permeation enhancers were formulated for sublingual delivery. Circular dichroism (CD) spectra indicated conformational stability, while chemical stability was confirmed by high-performance liquid chromatography (HPLC). CD spectra of insulin in combination with permeation enhancers revealed attenuation of molar ellipticity at 274 nm in the order TCTP > TC-AOT > TC > TC-NMT > Sol P > insulin solution. The molar ellipticity ratios at 208/222 nm confirmed dissociation of insulin in the microemulsions with the same rank order. Matrix-assisted laser diffraction ionization mass spectra (MALDI) revealed a significant shift in intensity signals towards monomer and dimers with a substantially high ratio of monomers, especially in the presence of the TCTP and TC-AOT. Permeation through porcine sublingual mucosa correlated with the dissociation data. A high correlation between the ratio of molar ellipticity at 208/222 nm and serum glucose levels (r (2) > 0.958) and serum insulin levels (r (2) > 0.952) strongly suggests the role of dissociation of insulin on enhanced absorption. While all microemulsions revealed a reduction in serum glucose levels and increase in serum insulin levels, significant differences were observed with the TCTP and TC-AOT microemulsions. High pharmacological availability >60 % and bioavailability >55 % compared to subcutaneous insulin at a low dose of 2 IU/kg appears highly promising. The data clearly suggests the additional role of the permeation enhancers on dissociation of insulin on enhanced sublingual absorption from the microemulsions.

Poly(ethylene glycol)-mediated conformational alteration of α-chymotrypsin prevents inactivation of insulin by stabilizing active intermediates

Proteolytic enzymes in the gut represent one of the biggest barriers against oral delivery of therapeutic proteins and peptides. In the current study, we explored the effect of poly(ethylene glycol) 400 (PEG 400), a commonly used crowding agent, on insulin degradation mediated by α-chymotrypsin (α-CT). Without PEG 400, insulin was quickly cleaved by α-CT to generate inactive degradation products. In comparison, incorporation of PEG 400 resulted in reaction mixtures with retained biological activity. The analysis on the conformation of α-CT and the local environment of the enzyme's active site unraveled that PEG 400 altered the conformation of α-CT to prevent the inactivation of insulin via stabilization of active intermediates. These findings indicated that PEG 400 may provide a promising addition toward oral delivery of insulin.

Polymeric hydrogels for oral insulin delivery

The search for an effective and reliable oral insulin delivery system has been a major challenge facing pharmaceutical scientists for over many decades. Even though innumerable carrier systems that protect insulin from degradation in the GIT with improved membrane permeability and biological activity have been developed, yet a clinically acceptable device is not available for human application. Efforts in this direction are continuing at an accelerated speed. One of the preferred systems widely explored is based on polymeric hydrogels that protect insulin from enzymatic degradation in acidic stomach and delivers effectively in the intestine. Swelling and deswelling mechanisms of the hydrogel under varying pH conditions of the body control the release of insulin. The micro and nanoparticle (NP) hydrogel devices based on biopolymers have been widely explored, but their applications in human insulin therapy are still far from satisfactory. The present review highlights the recent findings on hydrogel-based devices for oral delivery of insulin. Literature data are critically assessed and results from different laboratories are compared.

Bioavailability and variability of biphasic insulin mixtures

Absorption of subcutaneously administered insulin is associated with considerable variability. Some of this variability was quantitatively explained for both soluble insulin and insulin suspensions in a recent contribution to this journal (Søeborg et al., 2009). In the present article, the absorption kinetics for mixtures of insulins is described. This requires that the bioavailability of the different insulins is considered. A short review of insulin bioavailability and a description of the subcutaneous depot thus precede the presentation of possible mechanisms associated with subcutaneous insulin degradation. Soluble insulins are assumed to be degraded enzymatically in the subcutaneous tissue. Suspended insulin crystals form condensed heaps that are assumed to be degraded from their surface by invading macrophages. It is demonstrated how the shape of the heaps affects the absorption kinetics. Variations in heap formation thus explain some of the additional variability associated with suspended insulins (e.g. NPH insulins) compared to soluble insulins. The heap model also describes how increasing concentrations of suspended insulins lead to decreasing bioavailability and lower values of Cmax. Together, the findings constitute a comprehensive, quantitative description of insulin absorption after subcutaneous administration. The model considers different concentrations and doses of soluble insulin, including rapid acting insulin analogues, insulin suspensions and biphasic insulin mixtures. The results can be used in both the development of novel insulin products and in the planning of the treatment of insulin dependent diabetic patients.

Encapsulation of insulin–cyclodextrin complex in PLGA microspheres: a new approach for prolonged pulmonary insulin delivery

The insulin administration by pulmonary route has been investigated in the last years with good perspectives as alternative for parenteral administration. However, it has been reported that insulin absorption after pulmonary administration is limited by various factors. Moreover, in the related studies one daily injection of long-acting insulin was necessary for a correct glycemic control. To abolish the insulin injection, the present study aimed to develop a new formulation for prolonged pulmonary insulin delivery based on the encapsulation of an insulin:dimethyl-beta-cyclodextrin (INS:DM-beta-CD) complex into PLGA microspheres. The molar ratio of insulin/cyclodextrin in the complex was equal to 1:5. The particles were obtained by the w/o/w solvent evaporation method. The inner aqueous phase of the w/o/w multiple emulsion contained the INS:DM-beta-CD complex. The characteristics of the INS:DM-beta-CD complex obtained were assessed by 1H-NMR spectroscopy and Circular Dichroism study. The average diameter of the microspheres prepared, evaluated by laser diffractometry, was 2.53 +/- 1.8 microm and the percentage of insulin loading was 14.76 +/- 1.1. The hypoglycemic response after intratracheal administration (3.0 I.U. kg(-1)) of INS:DM-beta-CD complex-loaded microspheres to diabetic rats indicated an efficient and prolonged release of the hormone compared with others insulin formulations essayed.

Quantitative estimation of the effects of bile salt surfactant systems on insulin stability and permeability in the rat intestine using a mass balance model

The oral delivery of peptides and proteins is compromised by chemical and proteolytic instability as well as by permeability limitations. The aim of this study was to delineate the relative contributions of simple bile salt and bile salt:fatty acid mixed micellar systems to protein stability vs permeability enhancement in the rat intestine. Insulin disappearance from the rat intestine was evaluated when administered in simple micellar systems of sodium cholate (NaC), sodium taurocholate (NaTC) and sodium glycocholate (NaGC), and in mixed micellar systems of these bile salts and linoleic acid (LA). In-vitro stability studies were used to evaluate the extent of insulin degradation in the different micellar systems. After correction for insulin degradation in all systems a mass balance model was used to estimate the fractions of insulin absorbed for all systems. Mass balance estimates for the extent of insulin absorption in control perfusion systems were consistent with previously reported predictions of the model for ileal insulin absorption. Mass balance estimates for NaGC suggested no significant effects on the fraction of insulin absorbed relative to control. However, insulin absorption was estimated to occur to a significantly greater extent for NaTC simple micellar systems and was coincident with increased permeability of the hydrophilic marker molecule PEG 4000. The mass balance model estimated higher fractions of insulin absorbed for all mixed micellar systems in line with enhanced plasma insulin levels and higher PEG 4000 permeabilities for these systems.

Effectiveness of pirotiodecane, absorption enhancer, on nasal absorption in rabbits

The absorption enhancing effect of 1-[2-(decylthio) ethyl] azacyclopentan-2-one (Pirotiodecane), on drug permeation across rabbit nasal mucosa was studied. The nasal epithelial mucosa was isolated from rabbit nasal septum and mounted in an Ussing chamber to allow for monitoring of the membrane resistance (Rm), and the permeation of fluorescein isothiocyanate-labeled dextran (FD-4, M.W. 4,400 Da). Treatment with 0.05, 0.1, and 0.2% Pirotiodecane for 60 min decreased Rm, and increased the cumulative amount of FD-4 permeated in a concentration-dependent manner, suggesting that Pirotiodecane possesses passively a disassembly of tight junction to enable the enhanced FD-4 permeation. The remarkable increase in plasma concentration of FD-4 was also observed in intranasal co-administration with 1% Pirotiodecane in rabbits. The Rm was virtually maintained after the removal of Pirotiodecane, although recovery of Rm was not seen. On the other hand, the increase in plasma concentration of FD-4 with intranasal co-administration of 1% Pirotiodecane in rabbits in vivo was not observed in FD-4 administration at 15-60 min after administration of 1% Pirotiodecane alone. It was concluded that Pirotiodecane possesses a relatively short absorption enhancing effect through nasal epithelial.

State of insulin self-association does not affect its absorption from the pulmonary route

This study is designed to compare and contrast the pulmonary absorption profiles of monomeric and hexameric insulin in the presence or absence of ethylene diamine tetraacetic acid (EDTA) or n-tetradecyl-beta-d-maltoside (TDM). The pulmonary absorption of two forms of insulin was studied by monitoring the changes in plasma insulin and glucose levels after intratracheal administration of monomeric or hexameric insulin into anesthetized rodents. EDTA or TDM was added to the formulation in order to evaluate if either of these agents has effects on the rate and extent of pulmonary absorption of monomeric and hexameric insulin. The biochemical changes that may occur after acute administration of TDM-based formulation have also been investigated by estimating lung injury markers in bronchoalveolar lavage fluid. A dose-dependent increase in the plasma insulin and decrease in plasma glucose levels was observed when increasing concentrations of hexameric or monomeric insulin were administered via the pulmonary route. Pulmonary administration of monomeric and hexameric insulin produced comparable absorption profiles in the presence or absence of EDTA or TDM. The bronchoalveolar lavage fluid analysis did not show differences in the levels of injury markers produced in TDM-treated rats and that produced in saline-treated rats, indicating no evidence for adverse effects of TDM in these short-term studies. Overall, in terms of rapidity of action and efficacy to reduce blood sugar, monomeric insulin did not provide advantages over hexameric insulin when administered via the pulmonary route.

Mechanisms for absorption enhancement of inhaled insulin by sodium taurocholate

The effects of sodium taurocholate (NaTC) on the absorption of inhaled insulin was investigated using both in vivo and in vitro experiments. The absolute bioavailability of insulin when given as a nebulized solution (0.6 mM) to anesthetized intubated beagle dogs was low (2.6+/-0.3%). However, when NaTC at different concentrations (2-32 mM) were included in the formulations the bioavailability increased and at 32 mM it was about nine times higher (23.2+/-4.4%) than for pure insulin. In a similar concentration interval (20-25 mM) NaTC decreased the transepithelial electrical resistance (TEER) across Caco-2 cell monolayers leading to an increased permeability of insulin. At higher concentrations (above 30 mM) the viability of the Caco-2 cells decreased and the insulin permeability increased dramatically. Furthermore, we show that NaTC in the concentration range 2-15 mM gradually decreases the aggregation state of insulin, i.e., produces mono- or dimeric insulin from hexameric insulin. In conclusion, NaTC increases the bioavailability of nebulized insulin, increases the permeability of insulin across Caco-2 cell monolayers, and decreases the aggregation state of insulin at similar concentrations. We suggest that the main mechanisms behind the absorption enhancement of inhaled insulin by NaTC are the production of insulin monomers and an opening of tight junctions between adjacent airway epithelial cells.

Nasal bioavailability of peptide T in rabbits: absorption enhancement by sodium glycocholate and glycofurol

The aim of the present study was to investigate the absolute nasal bioavailability of Peptide T from aqueous formulations containing sodium glycocholate, an absorption enhancer with known effect on epithelial tight junctions, and/or glycofurol in a crossover study in rabbits. Additionally, the reversibility of the absorption enhancing effect of sodium glycocholate was studied by applying enhancer and peptide T with different time intervals and calculating Area Under the Curve of the peptide in plasma. It was shown that the bioavailability of Peptide T was significantly enhanced when glycofurol or sodium glycocholate was added to a nasal formulation. The nasal bioavailability of Peptide T in water (control formulation), 5% glycofurol, 5% glycofurol+1% sodium glycocholate and 1% sodium glycocholate was 5.9, 22, 29 and 59%, respectively. As indicated by the differences in t(max), C(max) and time-concentration profiles different patterns of Peptide T absorption were seen from the vehicles containing glycofurol and sodium glycocholate. In the reversibility study, the enhancing effect of sodium glycocholate on nasal absorption of Peptide T was found to be reversible within 4 h. It was concluded, that nasal absorption of Peptide T in rabbits was effectively enhanced by co-administration of sodium glycocholate, which also provided very fast absorption rates as well as a relatively short lasting effect of the absorption enhancing effect. Co-administration of glycofurol leads to enhanced and prolonged absorption of the peptide. Combining the two enhancers did not lead to increased peptide T absorption compared to 5% glycofurol alone.

The effect of absorption enhancers on the initial degradation kinetics of insulin by alpha-chymotrypsin

The goal of this investigation was to establish a fast method to screen various insulin absorption enhancers by following their effect on the initial kinetics of insulin incubated with alpha-chymotrypsin at 37 degrees C. A simple, sensitive and reproducible reversed phase high performance liquid chromatography (HPLC) method has been developed to carry out this goal. Linear responses (r > 0.999) were observed over the range of 0.4-4 U/ml for insulin. There was no significant difference (P < 0.05) between inter- and intra-day studies for insulin. The mean relative standard deviations (RSD%) of the results of within-day precision and accuracy of insulin were 12%. The assay was sensitive to detect the existence of any metabolite due to the addition of any absorption enhancers, even if it was not seen with insulin alone. Three metabolites (A-C) were detected only when insulin was incubated with alpha-chymotrypsin at 37 degrees C. Metabolite D was observed when either glycocholic acid (0.5, 1%) or taurochenodeoxycholate (0.5, 1%) was incubated with insulin in the absence of alpha-chymotrypsin at 37 degrees C. The compounds that significantly increased insulin T50% were glycyrrhizic acid (0.5%) > deoxycholic acid (1%) > deoxycholic acid (0.5%) > glycyrrhizic acid (1%) > cholic acid (0.5, 1%). Capric acid (0.5%), hydroxypropyl-alpha-cyclodextrin (0.5, 1%) and dimethyl-alpha-cyclodextrin (0.5, 1, 5%) did not significantly affect insulin T50%. The bile salts increased insulin T50% in this order: deoxycholate > cholate > glycocholate > taurocholate > taurodeoxycholate > taurochenodeoxycholate > glycodeoxycholate. The results obtained would support the feasibility of utilizing such method for screening any compound incorporated in insulin formulation. These compounds should be used in the minimum possible concentration to avoid or minimize insulin degradation.

Enhanced permeability of insulin across the rat intestinal membrane by various absorption enhancers: their intestinal mucosal toxicity and absorption-enhancing mechanism of n-lauryl-beta-D-maltopyranoside

We have examined the in-vitro permeability characteristics of insulin in the presence of various absorption enhancers across rat intestinal membranes and have assessed the intestinal toxicity of the enhancers using an in-vitro Ussing chamber method. The absorption enhancing mechanism of n-lauryl-beta-D-maltopyranoside was studied also. The permeability of insulin across the intestinal membranes was low in the absence of absorption enhancers. However, the permeability was improved in the presence of enhancers such as sodium glycocholate and sodium deoxycholate in the jejunum, and sodium glycocholate, sodium deoxycholate, n-lauryl-beta-D-maltopyranoside, sodium caprate and ethylenediaminetetraacetic acid (EDTA) in the colon. Overall, the absorption enhancing effects were greater on the colonic membrane than on the jejunal membrane. The intestinal membrane toxicity of these enhancers was characterized using the release of cytosolic lactate dehydrogenase from the colonic membrane. A marked increase in the release of lactate dehydrogenase was observed in the presence of sodium deoxycholate and EDTA. The release of lactate dehydrogenase in the presence of these absorption enhancers was similar to that seen with sodium dodecyl sulphate (SDS), used as a positive control, indicating high toxicity of these enhancers to the intestinal membrane. In contrast, sodium glycocholate and sodium caprate caused minor releases of lactate dehydrogenase, similar to control levels, suggesting low toxicity. In addition, the amount of lactate dehydrogenase in the presence of n-lauryl-beta-D-maltopyranoside was much less than that seen with sodium deoxycholate, EDTA and SDS. Therefore, sodium glycocholate, sodium caprate and n-lauryl-beta-D-maltopyranoside are useful absorption enhancers due to their high absorption enhancing effects and low intestinal toxicity. To investigate the absorption enhancing mechanisms of n-lauryl-beta-D-maltopyranoside, the transepithelial electrical resistance (TEER), voltage clamp experiments and the circular dichroism spectra were studied. n-Lauryl-beta-D-maltopyranoside decreased the TEER values in a dose-dependent manner, suggesting that the enhancer may open the tight junctions of the epithelium, thereby increasing the permeability of insulin via a paracellular pathway. This speculation was supported by the findings that 20 mM n-lauryl-beta-D-maltopyranoside produced a greater increase in the paracellular flux rate than in the transcellular flux rate by the voltage clamp studies. Evaluating the circular dichroism spectra we found that insulin oligomers were not dissociated to monomers by the addition of n-lauryl-beta-D-maltopyranoside, but dissociation did occur with the addition of sodium glycocholate. Thus, the dissociation of insulin was not a major factor in the absorption enhancing effect of n-lauryl-beta-D-maltopyranoside. These findings provide basic information to select the optimal enhancer for the intestinal delivery of peptide and protein drugs including insulin.

Permeation of unfolded basic fibroblast growth factor (bFGF) across rabbit buccal mucosa--does unfolding of bFGF enhance transport?

PURPOSE

To investigate whether recombinant human basic fibroblast growth factor (rhbFGF) would permeate freshly-excised rabbit buccal mucosa. In addition, the effect of a permeation enhancer (Na+ glycocholate) and the possibility of reversibly unfolding the globular protein to a more linear conformation to increase the permeability of the test protein was evaluated.

METHODS

The in vitro flux of bFGF through freshly-excised rabbit buccal mucosa was determined using side-by-side diffusion systems. Detection of bFGF was performed using gradient elution, reversed-phase high-pressure liquid chromatography (RP-HPLC). Fluorescence spectroscopy and heparin affinity chromatography were used to assess the tertiary structure of bFGF.

RESULTS

Preliminary in vitro results have demonstrated that the bFGF flux increased from 1.4 +/- 0.13 ng min-1 cm-2 to 3.2 +/- 0.38 ng min-1 cm-2 with the addition of 15 mM Na+ glycocholate (NaG) to the donor solution. Subsequent addition of guanidine HCl (GnHCl) to the donor solution (3 M) was not followed by a further increase in the flux of bFGF (2.9 +/- 0.26 ng min-1 cm-2). However, when the order of addition of the additives was reversed (GnHCl first followed by NaG), the flux of bFGF across rabbit buccal mucosa was increased. Upon addition of GnHCl, there was a significant (p < .05) increase in bFGF flux from 1.2 +/- 0.15 ng min-1 cm-2 to 5.0 +/- 0.58 ng min-1 cm-2. Addition of NaG further increased the flux to 8.5 +/- 1.1 ng min-1 cm-2 which was approximately 3- to 3.5-fold greater than that determined with the protein alone in the absence of any donor phase additives. The percent of parent bFGF remaining following a 3-hr exposure of a bFGF solution to either the mucosal, serosal, or both sides of rabbit buccal mucosa were 54.3 +/- 5.7%, 71.8 +/- 6.3%, and 36.2 +/- 5.4%, respectively with the majority of parent bFGF lost during the first 15 minutes. A model endopeptidase (endoproteinase Arg-C from mouse submaxillary gland) was shown in vitro to contribute to the loss in parent bFGF.

CONCLUSIONS

The permeation of bFGF across rabbit buccal mucosa may be significantly increased by initially unfolding the protein with GnHCl and then treating the tissue with the permeation enhancer, NaG. Refolding and possible reactivation of bFGF's bioactivity may occur following membrane transport and subsequent dilution into an infinite sink.

Stability of acyl derivatives of insulin in the small intestine: relative importance of insulin association characteristics in aqueous solution

The stability of insulin and its acyl derivatives in the small intestine was examined in vitro. When these compounds were incubated in small intestinal fluid at 37 degrees C, proteolysis of monoacyl insulins was reduced by increasing the carbon number of the fatty acid attached to Phe-B1 of the insulin molecule. In contrast, Phe-B1 and Lys-B29 diacylated insulins were more susceptible to hydrolysis than native insulin. Similar results were obtained using homogenates of the small intestinal mucosa, although the extent of the contribution of acylation to insulin degradation differed. The mechanism of the accelerated insulin proteolysis by diacylation was studied by circular dichroism (CD). The negative maxima at 270 nm in the CD spectra were attenuated for the diacyl derivatives, indicating that insulin association was inhibited by diacylation. Therefore, the increased proportion of monomers available for insulin proteolysis represents a main factor that makes diacyl derivatives unstable.

Cyclodextrins as mucosal absorption promoters of insulin. II. Effects of beta-cyclodextrin derivatives on alpha-chymotryptic degradation and enteral absorption of insulin in rats

The relative effectiveness of two beta-cyclodextrin derivatives, i.e., dimethyl-beta-cyclodextrin (DM beta CD) and hydroxypropyl-beta-cyclodextrin (HP beta CD), in enhancing enteral absorption of insulin was evaluated in the lower jejunal/upper ileal segments of the rat by means of an in situ closed loop method. The incorporation of 10% (w/v) DM beta CD to a 0.5 mg/ml porcine-zinc insulin solution dramatically increased insulin bioavailability from a negligible value (approximately 0.06%) to 5.63%, when administered enterally at a dose of 20 U/kg. However, addition of 10% (w/v) HP beta CD did not improve enteral insulin uptake significantly with a bioavailability of only 0.07%. Similarly, the pharmacodynamic relative efficacy values obtained after the enteral administration of 20 U/kg insulin, 20 U/kg insulin with 10% HP beta CD, and 20 U/kg insulin with 10% DM beta CD were 0.24%, 0.26%, and 1.75%, respectively. Biodegradation studies of 0.5 mg/ml insulin hexamers by 0.5 microM alpha-chymotrypsin revealed no inhibitory effect on the enzymatic activity by the two cyclodextrins. On the contrary, the apparent first-order rate constant increased significantly in the presence of 10% DM beta CD, suggesting insulin oligomer dissociation by DM beta CD. Histopathological examination of the rat intestine was performed to detect tissue damage following enteral administration of the beta-cyclodextrin derivatives. Light microscopic inspection indicated no observable tissue damage, thereby arguing direct membrane fluidization as the primary mechanism for enhanced insulin uptake. This study indicates the feasibility of using cyclodextrins as mucosal absorption promoters of proteins and peptide drugs.

Bile salt-fatty acid mixed micelles as nasal absorption promoters. III. Effects on nasal transport and enzymatic degradation of acyclovir prodrugs

The absorption enhancement and presystemic degradation kinetics of a homologous series of acyclovir 2'-ester prodrugs were investigated in rats using the in situ nasal perfusion technique in the presence of bile salt-fatty acid mixed micelles. In vitro incubation studies indicated that nasal perfusate containing a mixed micellar solution generated higher ester-cleaving activity than isotonic phosphate buffer washings. Inhibitor screening and substrate specificity studies demonstrated the enzyme to be most likely carboxylesterase rather than true cholinesterase. The extent of prodrug cleavage by the carboxylesterase appears to correlate well with the substrate lipophilicity for esters with linear acyl chains. On the other hand, branching of the acyl side chain significantly retards acyclovir prodrug breakdown. To estimate the nasal epithelial membrane and cytoplasmic damaging effect caused by sodium glycocholate (NaGC)-linoleic acid (15 mM:5 mM) mixed micelles, the release profiles of 5'-nucleotidase (5'-ND), lactate dehydrogenase (LDH), and carboxylesterase in the nasal perfusate were measured as a function of time. The results indicated that the activities of all three enzymes resulting from the mixed micellar solution appeared to be significantly higher than those caused by 15 mM NaGC alone. The apparent nasal absorption rate constants of acyclovir and its butyrate, valerate, pivalate, and hexanoate ester prodrugs in mixed micellar solutions containing an esterase inhibitor (1 mM phenylmethylsulfonyl fluoride) were individually calculated. Without an inhibitor, lengthening of the linear acyl side chain of the prodrug resulted in greatly accelerated degradation coupled with moderate absorption improvement. The solubilities and micellar binding constants of acyclovir prodrugs were also determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of anionic, cationic, nonionic, and physiologic surfactants on the dissociation, alpha-chymotryptic degradation, and enteral absorption of insulin hexamers

Various surfactants were investigated to compare their effects on insulin dissociation, alpha-chymotryptic degradation, and rat enteral absorption. With a circular dichroism technique, sodium dodecyl sulfate (SDS) at a 5 mM concentration was found to completely dissociate porcine-zinc insulin hexamers (0.5 mg/ml) into monomers. The catalytic activity of alpha-chymotrypsin (0.5 microM) was also abolished by 5 mM SDS. When insulin was injected into the distal jejunum/proximal ileum segment of the rat, 5 mM SDS greatly enhanced its pharmacological availability, from a negligible value to 2.8%. Being a cationic surfactant, hexadecyl trimethylammonium bromide (CTAB) also efficiently dissociated insulin hexamers at concentrations of 1-5 mM. However, extensive charge-charge interaction was observed below a CTAB concentration of 0.6 mM, leading to insulin precipitation at a molar CTAB:insulin ratio of 1:1 to 2:1. An alpha-chymotryptic degradation study also revealed near-complete dissociation of insulin hexamers at 1 mM CTAB. Above 1 mM, however, CTAB acted as an enzyme inhibitor, most likely by means of charge repulsion. Enteral absorption studies showed a much lower pharmacological availability, only 0.29%. Nonionic surfactants such as Tween 80 and polyoxyethylene 9 lauryl ether were ineffective in dissociating insulin hexamers. Tween 80, at 5 mM, neither significantly altered the alpha-chymotryptic degradation pattern nor enhanced the enteral absorption of insulin. The relative effectiveness of different species of bile salts on insulin hexamer dissociation appeared to be similar. Sodium glycocholate at a 30 mM concentration also significantly increased insulin pharmacological availability, to 2.3%. A morphological study did not reveal any significant alteration of the rat intestinal mucosal integrity after exposure to 5 mM SDS for 30 min.2+ transport.

Minimization of shaking-induced formation of insoluble aggregates of insulin by cyclodextrins

Aggregation is known to complicate insulin delivery and the processing and formulation of biotechnology-derived peptide/protein drugs. Shaking-induced formation of insoluble aggregates in bovine insulin and the potential role of cyclodextrins in preventing such aggregation were investigated. Insulin, dissolved in phosphate buffer, pH 7.2, and preserved with 2 mg/ml of phenol was aggregated, in triplicate, by shaking at 450 rpm for 2.5 days on a gyratory shaker. Visible aggregation was quantitated by measuring optical density in the visible range on a spectrophotometer. Solutions were then filtered through a 0.22 mu filter and the amount of insulin remaining in filtrate was determined by HPLC. Aggregation increased at lower concentrations, with solutions turning milky at 0.5 mg/ml; HPLC assay of filtrate indicated a complete loss of insulin. Under the same conditions, except for shaking, control solutions exhibited no insulin loss, excluding absorption as a cause of the insulin loss. The use of cyclodextrins (0.5 mg/ml) to stabilize insulin was investigated. alpha-, beta-, gamma- and hydroxypropyl-beta-cyclodextrin, each at 1.5% level, were used to prevent aggregation. The efficacy of cyclodextrins in preventing aggregation (% insulin aggregated in parentheses), was: hydroxypropyl-beta- (15) approximately beta- (18) > alpha- (54). No protection was observed with gamma-cyclodextrin.

Cyclodextrins as nasal absorption promoters of insulin: mechanistic evaluations

The safety and effectiveness of cyclodextrins (CD) as nasal absorption promoters of peptide-like macromolecules have been investigated. The relative effectiveness of the cyclodextrins in enhancing insulin nasal absorption was found to be in the descending order of dimethyl-beta-cyclodextrin (DM beta CD) greater than alpha-cyclodextrin (alpha-CD) greater than beta-cyclodextrin (beta-CD), hydroxypropyl-beta-cyclodextrin (HP beta CD) greater than gamma-cyclodextrin (gamma-CD). A direct relationship linking absorption promotion to nasal membrane protein release is evident, which in turn correlates well with nasal membrane phospholipid release. The magnitude of the membrane damaging effects determined by the membrane protein or phospholipid release may provide an accurate, simple, and useful marker for predicting safety of the absorption enhancers. In order to estimate further the magnitude of damage and specificity of cyclodextrin derivatives in solubilizing nasal membrane components, the enzymatic activities of membrane-bound 5'-nucleotidase (5'-ND) and intracellular lactate dehydrogenase (LDH) in the perfusates were also measured. HP beta CD at a 5% concentration was found to result in only minimal removal of epithelial membrane proteins as evidenced by a slight increase in 5'-ND and total absence of LDH activity. On the other hand, 5% DM beta CD caused extensive removal of the membrane-bound 5'-ND. Moreover, intracellular LDH activity in the perfusate increased almost linearly with time. The cyclodextrins are also capable of dissociating insulin hexamers into smaller aggregates, and this dissociation depends on cyclodextrin structure and concentration. Enhancement of insulin diffusivity across nasal membrane through dissociation may provide an additional mechanism for cyclodextrin promotion of nasal insulin absorption.