Arg-Gly-Asp (RGD) peptides contain an aspartic acid residue that is highly susceptible to chemical degradation and leads to the loss of biological activity. Our hypothesis is that cyclization of RGD peptides via disulphide bond linkage can induce structural rigidity, thereby preventing degradation mediated by the aspartic acid residue. In this paper, we compared the solution stability of a linear peptide (Arg-Gly-Asp-Phe-OH; 1) and a cyclic peptide (cyclo-(1, 6)-Ac-Cys-Arg-Gly-Asp-Phe-Pen-NH2; 2) as a function of pH and buffer concentration. The decomposition of both peptides was studied in buffers ranging from pH 2-12 at 50 degrees C. Reversed-phase HPLC was used as the main tool in determining the degradation rates and pathways of both peptides. Fast atom bombardment mass spectrometry (FAB-MS), electrospray ionization mass spectrometry (ESI-MS), matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry, liquid chromatography-mass spectrometry (LC-MS), and one- and two-dimensional nuclear magnetic resonance spectroscopy (NMR) were used to characterize peptides 1 and 2 and their degradation products. In addition, co-elution with authentic samples was used to identify degradation products. Both peptides displayed pseudo-first-order kinetics at all pH values studied. The cyclic peptide 2 appeared to be 30-fold more stable than the linear peptide 1 at pH 7. The degradation mechanisms of linear (1) and cyclic (2) peptides primarily involved the aspartic acid residue. However, above pH 8 the stability of the cyclic peptide decreased dramatically due to disulphide bond degradation. Both peptides also exhibited a change in degradation mechanism upon an increase in pH. The increase in stability of cyclic peptide 2 compared to linear peptide 1, especially at neutral pH, may be due to decreased structural flexibility imposed by the ring. This rigidity would prevent the Asp side chain carboxylic acid from orientating itself in the appropriate position for attack on the peptide backbone.

Peptides containing the Arg-Gly-Asp (RGD) sequence can inhibit platelet aggregation. Incorporation of this sequence into a cyclic peptide results in specific binding to a particular integrin. Studies of cyclic RGD peptides show that residues surrounding the RGD sequence have important effects on the selectivity of the peptide to bind with glycoprotein IIb/IIIa (GPIIb/IIIa). In this paper, we elucidate the conformation of cyclo(2,10)Ac-Gly1-Pen2-Gly3-His4-Arg5-Gly6-Asp7 -Leu8-Arg9-Cys10-Ala11-NH2 (1) by NMR and molecular dynamics simulations. This peptide inhibits platelet aggregation in a manner similar to that reported for cyclo(2,10)Gly1-Pen2-Gly3-His4-Arg5-Gly6-Asp7-Le u8-Arg9-Cys10-Ala11-OH (6) (Cheng, S. et al. J. Med. Chem. 1994, 37, 1-8), which is shown to be selective for the GPIIb/IIIa receptor. The cyclic peptide 1 exhibited a major and a minor conformer in solution. In the major conformer, the His4-Arg5-Gly6-Asp7 segment encompasses a 4-->1 hydrogen bond with a distorted type II beta-turn, and the minor conformer has turn-extended-turn. A comparison between the major conformation of this peptide and those of other cyclic RGD peptides suggests the importance of a hydrophobic residue adjacent to the RGD sequence.

The dissolution rate of a solid drug from the gastrointestinal (GI) tract is affected by the properties and flow dynamics of the liquid medium surrounding the tablet, as well as by the chemical nature of the drug. In this study, naproxen was used as a poorly soluble model drug. The dissolution medium was buffered with acetate, citrate, or phosphate buffer of varied concentrations and pH. GI flow conditions around a stationary tablet were simulated in a laminar flow device by anchoring the tablet on the floor of its channel having a rectangular cross section. Fresh, buffered solution was passed across the tablet and the effluent was collected for analysis and calculation of the dissolution rate. The dissolution rate was found to vary nonlinearly with the exposed tablet height, reaching a maximum at a tablet height approximately half the channel height. This maximum rate was attributed to an optimal combination of (1) eddy mixing and local turbulence generated by the flow impingement on the bluff object (tablet) and (2) the exposed tablet surface area available for dissolution. This effect was further confirmed by using dye-enhanced visual analysis of flow patterns at varied flow rates and exposed tablet heights. Elevation of the tablet to approximately the channel half-height significantly magnified the dissolution rate increase observed on exposure to buffered medium. Thus, tablet height and exposed surface area are major factors in determining dissolution rate, especially in conditions where the dissolving species reacts with the solvent. These results suggest that standard in vitro dissolution rate methods do not qualitatively indicate incremental changes in rate with altered tablet geometry or dissolution medium.

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The objective of this study was to determine the primary formulation properties that affect the dissolution rate of poorly soluble nondisintegrating drugs. This work focused on compression of orally administered acidic drugs with ionizable buffers. Naproxen was used as the poorly soluble model drug, and calcium salts of carbonic, citric, and phosphoric acids were used as formulation buffers. Gastrointestinal tract (GI) dissolution was simulated in a laminar flow apparatus by exposing a drug/buffer tablet to aqueous solution of a given pH at a constant flow rate. Although formulation with a buffer resulted in reduced available drug surface area, the absolute drug dissolution rate and flux increased with increased buffer content to a maximum from tablets having equal weights of drug and buffer. This buffer-induced enhancement was seen not only in GI tract simulation (pH 7), but also at pH 2 (stomach conditions), where acidic drugs remain in their poorly soluble form upon dissolution. The flux increase was much greater than that achieved by using the same amount of an inert excipient in the solid formulation. Dissolution rates were also increased by decreased drug and buffer particle sizes and increased fluid flow rate. Drug dissolution rates were inversely proportional to intrinsic buffer solubilities: The model drug actually dissolved fastest when the buffer solubility was lower than that of the drug. Dissolution rates were apparently insensitive to the relative proximity of the drug and buffer ionization constants.