Mechanistic roles of neutral surfactants on concurrent polarized and passive membrane transport of a model peptide in Caco-2 cells

The transport of the model peptide Acf(NMef)2NH2 across Caco-2 cell monolayers was studied in the apical (AP) to basolateral (BL) and the BL to AP direction in the presence of Polysorbate 80 or Cremophore EL in the AP compartment. Increasing surfactant concentrations resulted in increasing AP-->BL peptide permeability and decreasing BL-->AP permeability. In either direction, limiting permeabilities were achieved at concentrations less than the critical micellar concentrations (cmc's) of the surfactants, and remained constant at much higher concentrations. These plateau permeabilities were not equivalent in the two directions. This residual assymetry was abolished by increasing the peptide concentration. Altogether, the observations support the presence of at least two pumps in Caco-2 cells for this peptide, polarized in the BL-->AP direction. These experimental results were analyzed within the context of a quantitative biophysical model incorporating concurrent passive diffusion across the AP and BL membranes accompanied by surfactant-inhibitable active polarized efflux across the AP membrane. The model was also used to locate the additional transport activity at the BL membrane as an uptake pump. Under conditions of complete inhibition, the intrinsic passive diffusional permeability of Acf(NMef)2NH2 was found to be 13 x 10(-6) cm/s, essentially identical with results reported earlier with this peptide utilizing verapamil as an inhibitor. With respect to the mechanism of surfactant inhibition of the apical efflux transport, the monomeric species was found to be responsible with no contribution from micelles. Modeling the mode of inhibition as a noncompetitive Michaelis-Menten process gave identical Kis of 0.5 microM for the two surfactants. Finally, increase of either surfactant beyond 750 microM resulted in a decrease of peptide permeability in the AP-->BL direction. This was attributed to weak association of the peptide with micelles in the AP compartment, which effectively decreased the thermodynamic activity of the peptide at surfactant concentrations greater than 20 times their cmc. Both the experimental approach and accompanying theoretical model demonstrated in this work will allow for further characterization of the inhibitory potencies of surfactants for the nonpassive efflux pathway in vitro and in vivo.

Selective fluorogenic derivatization of a peptide nucleic acid trimer with naphthalene-2,3-dicarboxaldehyde

The reversed-phase high-performance liquid chromatography of a Peptide Nucleic Acid (PNA) trimer has been studied after its preseparation fluorogenic derivatization with naphthalene-2,3-dicarboxaldehyde in the presence of cyanide (NDA/CN). Trace levels of the PNA trimer were determined in cell homogenate samples containing the PNA trimer at prederivatization concentrations as low as 48.9 ng ml-1. The sample pretreatment operations included a deproteination step, achieved by ultra-filtration, followed by fluorogenic derivatization (NDA/CN). Subsequently, to achieve adequate selectivity, the fluorescently labeled PNA was subjected to high performance anion exchange chromatography prior to quantitation via fluorescence detection. The various problems encountered during sample pretreatment and separation of derivatized PNA trimer in biological samples are presented and discussed.

The use of surfactants to enhance the permeability of peptides through Caco-2 cells by inhibition of an apically polarized efflux system

PURPOSE

It has recently been reported that the permeability of peptides across Caco-2 cells, an in vitro model of the intestinal mucosa, was limited by an apically polarized efflux mechanism. Since surfactants (e.g. Cremophor EL, Polysorbate 80) have been reported to inhibit similar efflux systems in tumor cells, we determined whether they could enhance the permeability of peptides across monolayers of Caco-2 cells.

METHODS

The transport studies of [3H]-mannitol and [14C]-model peptides were carried out across the Caco-2 cell monolayers. TEER values were determined using Voltohmmeter with STX-2 electrode and the equilibrium dialysis studies were conducted using side-by-side dialysis apparatus with cellulose ester membranes.

RESULTS

Initially, [3H]-mannitol flux studies were conducted to find concentrations of the surfactants that did not cause damage to the cell monolayer. Based on these studies, Polysorbate 80 and Cremophor EL were selected for further study. The fluxes of [14C]-AcfNH2 (a nonsubstrate for this efflux system) and [14C]-Acf(N-Mef)2NH2 (a substrate for this efflux system) were then measured in the absence and presence of the two surfactants. The permeability of [14C]-AcfNH2 was not affected by the surfactants, while that of [14C]-Acf(N-Mef)2NH2 increased with increasing concentrations of surfactants and then decreased. For example, the Pe values for [14C]-Acf(N-Mef)2NH2 were 3.75 x 10(-6), 8.58 x 10(-6), 10.29 x 10(-6), 7.48 x 10(-6), and 1.46 x 10(-6) cm/sec with Cremophor EL concentrations of 0, 0.01, 0.1, 1 and 10% w/v, respectively. This bimodal effect of surfactants on the Caco-2 cell permeability of this peptide was shown to be due to the interactions between the peptide and micelles at higher concentrations of surfactants, which were demonstrated by the equilibrium dialysis experiments.

CONCLUSIONS

These results suggest that surfactants, which are commonly added to pharmaceutical formulations, may enhance the intestinal absorption of some drugs by inhibiting this apically polarized efflux system.