Enabling the intestinal absorption of highly polar antiviral agents: ion-pair facilitated membrane permeation of zanamivir heptyl ester and guanidino oseltamivir

Abstract

Antiviral drugs often suffer from poor intestinal permeability, preventing their delivery via the oral route. The goal of this work was to enhance the intestinal absorption of the low-permeability antiviral agents zanamivir heptyl ester (ZHE) and guanidino oseltamivir (GO) utilizing an ion-pairing approach, as a critical step toward making them oral drugs. The counterion 1-hydroxy-2-naphthoic acid (HNAP) was utilized to enhance the lipophilicity and permeability of the highly polar drugs. HNAP substantially increased the log P of the drugs by up to 3.7 log units. Binding constants (K(11(aq))) of 388 M(-1) for ZHE-HNAP and 2.91 M(-1) for GO-HNAP were obtained by applying a quasi-equilibrium transport model to double-reciprocal plots of apparent octanol-buffer distribution coefficients versus HNAP concentration. HNAP enhanced the apparent permeability (P(app)) of both compounds across Caco-2 cell monolayers in a concentration-dependent manner, as substantial P(app) (0.8-3.0 x 10(-6) cm/s) was observed in the presence of 6-24 mM HNAP, whereas no detectable transport was observed without counterion. Consistent with a quasi-equilibrium transport model, a linear relationship with slope near 1 was obtained from a log-log plot of Caco-2 P(app) versus HNAP concentration, supporting the ion-pair mechanism behind the permeability enhancement. In the rat jejunal perfusion assay, the addition of HNAP failed to increase the effective permeability (P(eff)) of GO. However, the rat jejunal permeability of ZHE was significantly enhanced by the addition of HNAP in a concentration-dependent manner, from essentially zero without HNAP to 4.0 x 10(-5) cm/s with 10 mM HNAP, matching the P(eff) of the high-permeability standard metoprolol. The success of ZHE-HNAP was explained by its >100-fold stronger K(11(aq)) versus GO-HNAP, making ZHE-HNAP less prone to dissociation and ion-exchange with competing endogenous anions and able to remain intact during membrane permeation. Overall, this work presents a novel approach to enable the oral delivery of highly polar antiviral drugs, and provides new insights into the underlying mechanisms governing the success or failure of the ion-pairing strategy to increase oral absorption.

Figure 1

Chemical structures of ZHE, GO, and HNAP. Ionization states at pH 6.5 as per pK_a values in Table 1 are shown for reference.

Figure 2

Double reciprocal plot of the apparent octanol-buffer (pH 6.5) distribution coefficient of GO as a function of HNAP concentration.

Figure 3

Double reciprocal plot of the apparent octanol-buffer (pH 6.5) distribution coefficient of ZHE as a function of HNAP concentration.

Figure 4

Top: Dependence of GO P_app on HNAP concentration in Caco-2 cell assay. Bottom: Log GO P_app across Caco-2 cell monolayers as a function of Log HNAP concentration. Linear relationship with a slope near 1 is consistent with an ion-pair mediated transport mechanism as per Equation 14.

Figure 5

Top: Dependence of ZHE P_app on HNAP concentration in Caco-2 cell assay Bottom: Log ZHE P_app across Caco-2 cell monolayers as a function of Log HNAP concentration. Linear relationship with a slope near 1 is consistent with an ion-pair mediated transport mechanism as per Equation 14.

Figure 6

GO P_eff versus time in rat jejunal perfusion assay at 0 mM and 12 mM HNAP concentrations

Figure 7

ZHE P_eff versus time in rat jejunal perfusion assay at 0, 4, and 10 mM HNAP concentrations

Scheme 1

Chemical Synthesis of Guanidino Oseltamivir

Scheme 2