BZD methylcarbamate compounds are insoluble or slightly soluble in water. Solubility studies involving ABZ
] or ABZSO
] indicate that BZD methylcarbamate compounds possesses both acidic and basic groups in their chemical structure, with pKa values ranging between 7.2-10.7 and 2.4-4.7, respectively. Thus, the aqueous solubility of these compounds increases greatly when pH values are above 11 or below 2. The impact of low pH values in improving the aqueous solubility of different BZD compounds has been previously demonstrated
]. In the same direction, higher aqueous solubility of different BZD molecules was observed under our in vitro
experimental conditions (Table
). A markedly higher aqueous solubility at both, pH 1.2 or 7.4, was obtained for ABZ, ABZSO, MBZ, OFZ and FLBZ. However, the highest solubility improvement was observed for FLBZ at pH 1.2, where the presence of HPßCD enhanced 70 fold its water solubility (Table
FLBZ, R-FLBZ and H-FLBZ were recovered in plasma samples after i.r. administration of FLBZ formulated as either the CDs-solution or CMC-suspension. The active anthelmintic molecule after FLBZ treatment is the parent drug, which exhibits a high affinity binding for ß-tubulin
] and high potency in parasite motility test
]. Additionally, the ovicidal activity against Fasciola hepatica
eggs of the R-FLBZ metabolite has recently been described
] as well as the in vitro
activity against Echinococcus granulosus
]. Additionally, this metabolite has demonstrated ability to accumulate (under ex vivo
conditions) into target parasites equivalent to that observed for the parent FLBZ compound
]. Consequently, a complementary effect of the parent drug and the reduced metabolite may account for the final anthelmintic activity after FLBZ administration.
The limited aqueous solubility of the most potent BZD compounds allows their formulation only as suspensions for oral administration to sheep. Water solubility and the rate of dissolution in the gastrointestinal (GI) tract lumen have been shown to be relevant to the absorption and resultant systemic availability of BZD compounds in ruminants
]. To increase the bioavailability of poorly soluble compounds, pharmaceutical scientists may rely on particle micronization, use of surfactant agents, or use of complexing agents such as CDs
]. Increased ABZ aqueous solubility induced by HPßCD has previously been reported
]. Moreover, the administration of an ABZ-HPßCD formulation to sheep increased GI absorption of ABZ with an increment (37%) in the AUC value of the active ABZSO metabolite
]. However, this formulation-based advantage in water solubility did not reach, under our experimental conditions, a beneficial pharmacokinetic result compared to the treatment with the FLBZ suspension. Contrarily to what was expected, the absorption-related pharmacokinetic parameters did not show any marked formulation-dependant effect. Only the Cmax value for the R-FLBZ metabolite was higher (P < 0.05) after administration of the novel CD-based preparation (0.23 ± 0.04
μg/mL) compared to that observed after the treatment with the CMC-suspension (0.14 ± 0.03
). The AUC values obtained after the administration of a CDs-based formulation resulted 73% (FLBZ) and 39% (R-FLBZ) higher than those obtained for the CMC-based formulation. However, these differences did not reach statistical differences due to the large variation among experimental animals.
In mice treated with the FLBZ-HPßCD solution, the enhanced FLBZ absorption resulted in a significantly higher (>450%) plasma Cmax compared to that obtained after the FLBZ-suspension treatment (P < 0.05)
]. This modified pharmacokinetic behaviour permitted a higher drug exposure of the hydatid cysts developed in mice, which enhanced the clinical efficacy of FLBZ in echinococcosis
]. The drastic changes induced by CDs in the systemic availability of FLBZ in mice, do not correlate with the results observed in sheep. Chemical stability of numerous foreign compounds is affected by the microflora in the rumen (the first forestomach in ruminants). CDs are extensively metabolized in the human colon
]. It is then likely that after the i.r. administration of the CD-FLBZ complex, a ruminal microflora-mediated metabolic process may have hydrolized/destroyed the CDs structure. This may facilitate FLBZ release into the ruminal fluid, which could then be associated to the particulate ruminal material as it occurs after the treatment with the suspension formulation. Such a phenomenon may account to explain the equivalent FLBZ systemic availability obtained following administration of two pharmaceutically well distinguish formulations. The absence of significant kinetic changes observed for FLBZ/metabolites in abomasal fluid after its administration as a CDs or CMC-formulation supports this argument.
To confirm the hypothesis of a CDs ruminal degradation, a pharmacokinetic study involving the i.a. administration of FLBZ as a CDs-based solution was performed. The plasma disposition kinetics of FLBZ/metabolites after its i.a. administration agrees with those reported after the i.a. administration of FBZ
] and ABZ
] in sheep. Basically, the AUC and the Tmax of the parent compound were significantly (P < 0.05) reduced by ruminal bypass. Furthermore, a significantly (P < 0.05) shorter T½abs for FLBZ compared to the i.r. administration was observed (Table
). Similar pharmacokinetic differences were observed for the R-FLBZ metabolite. When an orally administered BZD suspension reaches the rumen, an extensive adsorption of BZD molecules to the rumen particulate digesta has been shown to occur shortly after treatment
]. This extensive association between drug molecules and the particulate material of the digesta does not inhibit absorption but delays the rate of passage of the drug down the GI tract. The rumen acts as a “drug reservoir” and prolongs the duration of drug absorption
]. After the direct i.a. administration of a conventional BZD suspension, the drug formulation may bypass the rumen and the advantage of the “ruminal reservoir effect” is lost, resulting in poor dissolution of the BZD suspension in the abomasum due to a shorter residence time at the acidic abomasal pH, which results in reduced intestinal absorption and lower systemic availability. After the i.a. administration of FLBZ as a HPßCD-based solution, we expected to observe an enhanced absorption and subsequent bioavailability of the parent compound. However, the CDs formulation behaved similar to the conventional suspension. The association (adsorption) of FLBZ with digestion particulate material could explain the lack of a clear effect on drug absorption induced by the novel formulation (FLBZ-CDs solution).