Identification of esterase involved in the metabolism of two corticosteroid soft drugs.

The soft drug approach is successful in obtaining high local therapeutic efficacy without systemic adverse effects, because soft drugs are designed to be bioconverted to inactive form by hydrolytic enzymes in systemic circulation. However, there is little information about the exact nature of these metabolic enzymes. In this study, the human enzymes for biotransformation of soft drugs were investigated. Loteprednol etabonate (LE) and etiprednol dicloacetate (ED) were designed from Δ1-cortienic acid (Δ1-CA), the inactive metabolite of prednisolone, by introducing two labile ester bonds to restore the corticosteroidal activity. We found that LE and ED were mainly deactivated in human plasma rather than the liver. Inactive monoesters were produced, but the second hydrolysis to Δ1-CA was much slower. ED was hydrolyzed 10 times faster than LE in plasma (t1/2=1.35±0.08, 12.07±0.52h respectively). Paraoxonase 1 that attached with high density lipoprotein (HDL) was found to be the major hydrolase for LE and ED in human plasma as demonstrated by enzyme inhibition and stimulation experiments and the hydrolysis in lipoproteins-rich plasma fractions. Human serum albumin (HSA) showed slight hydrolase activity against ED but not LE. LE was slowly hydrolyzed in liver (clearance: 0.21±0.04 and 2.41±0.13ml/h/kg in liver and plasma, respectively) but ED wasn't hydrolyzed at all, so LE has superior metabolism in two sites. The difficult diffusion of HDL into tissues from blood suggests the stable presence of LE at the administration site, while ED might be deactivated by its relatively rapid chemical hydrolysis and hydrolase activity of HSA, in the interstitial fluid of the administration tissue. Moreover, deactivation in plasma and strong protein binding (around 98%) minimize the adverse effects of LE and ED in the systemic circulation.

Development of loteprednol etabonate-loaded cationic nanoemulsified in-situ ophthalmic gel for sustained delivery and enhanced ocular bioavailability.

A novel cationic nanoemulsified in-situ ophthalmic gel of loteprednol etabonate (LE) was developed to improve the permeability and retention time of formulations for overall improvement of drug's ocular bioavability. Capryol 90 (oil phase), tween 80 (surfactant) and transcutol P (cosurfactant) was selected as formulation excipients to construct pseudoternary phase diagrams and nanoemulsion region was recognized from diagrams. Spontaneous emulsification method was used to manufacture LE nanoemulsion and it was optimized using 32 factorial design by considering the amount of oil and the ratio of surfactant to cosurfactant (Smix) as independent variables and evaluated for various physicochemical properties. Optimized NE was dispersed in Poloxamer 407 and 188 solution to form nanoemulsified sols that were predictable to transform into in-situ gels at corneal temperature. Drug pharmacokinetics of sterilized optimized in situ NE gel, NE-ISG2 [0.69% w/w Capryol 90, 0.99%w/w Smix (3:1), 13% Poloxamer 407, 4% w/w Poloxamer 188] and marketed formulation were assessed in rabbit aqueous humor. The in-situ gels were clear, shear thinning in nature and displayed zero-order drug release kinetics. NE-ISG2 showed the minimum ocular irritation potential and significantly (p < 0.01) higher Cmax and AUC(0-10 h), delayed Tmax, extended mean residence time and improved (2.54-fold times) bioavailability compared to marketed formulation.