Mutual Inhibition of the Insulin Absorption-Enhancing Properties of Dodecylmaltoside and Dimethyl--Cyclodextrin Following Nasal Administration Fakhrul Ahsan, 1 John J. Arnold, 1 Elias Meezan, 1 and Dennis J. Pillion 1,2 Received November 13, 2000; accepted January 19, 2001 Purpose. To determine if a nasal insulin formulation containing two distinct absorption-enhancing agents exhibits an additive or synergis- tic increase in the rate of systemic insulin absorption. Methods. The pharmacokinetics and pharmacodynamics of insulin absorption were measured in hyperglycemic anesthetized rats follow- ing nasal insulin administration with formulations containing two dif- ferent types of absorption-promoting agents, dimethyl--cyclodextrin (DMBCD) and dodecylmaltoside (DDM). Results. When either DDM (0.1–0.5%) or DMBCD (1.0–5.0%) was added to the nasal insulin formulation, a significant and rapid in- crease in plasma insulin levels was observed, with a concomitant decrease in blood glucose concentration. A combined preparation containing 0.25% DDM (0.005 M) and 2.5% DMBCD (0.019M), however, failed to cause an increase in plasma insulin levels or a decrease in blood glucose concentration. Increasing concentrations of DDM added to an insulin formulation with a fixed DMBCD concen- tration caused a decrease, rather than an increase, in systemic ab- sorption of insulin. Conclusions. Mixing DMBCD and DDM resulted in mutual inhibi- tion of their ability to enhance systemic absorption of insulin follow- ing nasal delivery. The results are consistent with the formation of an inclusion complex between DDM and DMBCD which lacks the abil- ity to enhance nasal insulin absorption. KEY WORDS: nasal insulin; dodecylmaltoside; dimethyl- - cyclodextrin; inclusion complex; pharmacokinetics; blood glucose. INTRODUCTION Cyclodextrins are cyclic oligosaccharides, composed of 6 or more glucose units with a characteristic central cavity, that have the ability to form inclusion complexes with hydropho- bic molecules (1). The most extensively studied cyclodextrins are -, -, and -cyclodextrins, which consist of six, seven, and eight glucopyranose units, respectively. Both natural and modified cyclodextrins are presently used in pharmaceutical formulations to increase drug solubility and dissolution, and enhance drug absorption by means of molecular encapsula- tion. Certain cyclodextrins have been found useful in enhanc- ing nasal absorption of peptide drugs including insulin (2). Among the cyclodextrin derivatives studied as nasal absorp- tion promoters, dimethyl--cyclodextrin (DMBCD) was found to be the most effective, while -cyclodextrin was less effective and - and -cyclodextrin had negligible effects on insulin absorption (2). The mechanism of action that pro- duces increased nasal absorption of peptide drugs is not clear. Cyclodextrins may protect peptide drugs from enzymatic deg- radation by molecular encapsulation or directly deactivate proteolytic enzymes; in particular, dimethyl--cyclodextrin may have a direct effect on the membrane and enhance drug absorption by binding with, and/or extracting/removing mem- brane components that serve as a barrier to insulin transport; or cyclodextrins may interact directly with hydrophobic side chains on the peptide drug molecules and change their inher- ent aggregability or permeability across a phospholipid bilayer (3–5). Loftsson and Jarvinen report that the ability of cyclodextrins to interact with biological membranes is greatly reduced when their cavity is occupied by a lipophilic substrate (6). Many other potential absorption promoters besides the cyclodextrins have been investigated (7–11). Studies in this laboratory have shown that alkylglycosides such as dodecyl- maltoside, dodecylsucrose, and tetradecylmaltoside are po- tent enhancers of insulin absorption following either ocular or nasal delivery (12,13). Dodecylmaltoside (DDM), a nonionic alkylglycoside containing the disaccharide maltose, glycosidi- cally linked to a twelve carbon alkyl chain, significantly in- creased nasal absorption of insulin when used even at very low concentrations (0.06–0.25%). The mechanism of action of DDM is not clear. It appears that DDM has a direct effect on the epithelium, rather than on the multimeric insulin mol- ecule, since it proved equally effective as an enhancer of the nasal absorption of multimeric regular human insulin and fast-acting monomeric lyspro insulin (14). Currently, it is not known if DMBCD will form an inclu- sion complex with DDM, and whether or not such a complex is still biologically active as an absorption enhancer. This study was designed to determine if DDM and DMBCD could be formulated together to provide greater insulin absorption at lower concentrations of surfactants than obtained with ei- ther absorption promoter alone. EXPERIMENTAL Materials DDM and DMBCD were purchased from Anatrace Corp. (Maumee, OH) and Sigma Chemicals (St Louis, MO), respectively. Regular human insulin (Humulin® 100 Units/ ml) was obtained from Eli Lilly & Company (Indianapo- lis, IN). Preparation of Nasal Formulations DDM or DMBCD stock solutions were prepared by dis- solving the excipients in normal saline and were stored for 30 days or less at 4°C. On the day of an experiment, the stock solutions were used to prepare the desired concentrations of the excipients. The concentrations used for DDM were 0.1% (2 mM), 0.25% (5 mM), 0.5% (10 mM), and 1% (20 mM), and the concentrations for DMBCD were 1.0% (7.5 mM), 2.5% (19 mM), 3.75% (28 mM), and 5.0% (38 mM). It should be noted that all concentrations of DDM used in this study were 1 Department of Pharmacology and Toxicology, School of Medicine, University of Alabama at Birmingham, 1670 University Boulevard, Birmingham, Alabama 35294-0019. 2 To whom correspondence should be addressed. (e-mail: dpillion@ uab.edu) Pharmaceutical Research, Vol. 18, No. 5, 2001 Research Paper 608 0724-8741/01/0500-0608$19.50/0 © 2001 Plenum Publishing Corporation