Juhani Huuskonen

Synthesis and in vitro/in vivo evaluation of novel oral N-alkyl- and N,N-dialkyl-carbamate esters of entacapone.

Entacapone has a relatively low oral bioavailability which may, in part, be due to its low aqueous solubility at low pH and/or its hydrophilic character at neutral pH. Various novel N-alkyl and N,N-dialkyl carbamate esters of entacapone were synthesized as possible prodrugs of entacapone in order to increase its aqueous solubility at an acidic pH and to increase its lipophilicity at neutral pH. Oral bioavailability of entacapone and selected carbamate esters were investigated in rats. Both N-alkyl and N,N-dialkyl carbamate esters were relatively stable against chemical hydrolysis at pH 7.4 (t1/2 = 14.9-20.7 h), but hydrolyzed rapidly (t1/2 = 0.8-2.7 h) in human serum. However, in contrast to N-alkyl carbamates, N,N-dialkyl carbamates did not release entacapone in in vitro enzymatic hydrolysis (human serum) studies. N-Alkyl carbamates, 2a-c, showed increased aqueous solubility at pH 7.4, of which 2a and 2c also show increased aqueous solubility at pH 5.0, compared to entacapone. In addition to increased aqueous solubility, 2c showed increased lipophilicity at pH 7.4. However, two N-alkyl carbamates of entacapone did not increase the oral bioavailability of the parent drug in rats. Thus, it can be concluded that the relatively low lipophilicity of entacapone is not the cause of its low bioavailability.

Synthesis of a water-soluble prodrug of entacapone

Entacapone was reacted with phosphorous oxychloride in dry pyridine to yield a phosphate ester. The phosphate promoiety increased aqueous solubility of the parent drug by more than 1700- and 20-fold at pH 1.2 and 7.4, respectively. The phosphate ester provides adequate stability (t(1/2) = 2227 h; pH 7.4) towards chemical hydrolysis, and allowed for release of the parent drug via enzymatic hydrolysis in liver homogenate.

Anandamide prodrugs. 1. Water-soluble phosphate esters of arachidonylethanolamide and R-methanandamide.

Phosphate esters of arachidonylethanolamide (AEA) and R-methanandamide were synthesized and evaluated as water-soluble prodrugs. Various physicochemical properties (pK(a), partition coefficient, aqueous solubility) were determined for the synthesized phosphate esters. The chemical stability of phosphate esters was determined at pH 7.4. In vitro enzymatic hydrolysis rates were determined in 10% liver homogenate, and in a pure enzyme-containing (alkaline phosphatase) solution at pH 7.4. The intraocular pressure (IOP) lowering properties of R-methanandamide phosphate ester were tested on normotensive rabbits. The phosphate promoiety increased the aqueous solubility of the parent compounds by more than 16500-fold at pH 7.4. Phosphate esters were stable in buffer solutions, but rapidly hydrolyzed to their parent compounds in alkaline phosphatase solution (t(1/2)<<15 s) and liver homogenate (t(1/2)=8-9 min). The phosphate ester of R-methanandamide reduced IOP in rabbits. These results indicate that the phosphate esters of AEA and R-methanandamide are useful water-soluble prodrugs.

Synthesis and in-vitro/in-vivo evaluation of orally administered entacapone prodrugs

Entacapone is a new inhibitor of catechol‐O‐methyltransferase (COMT) that is used as an adjunct to l‐dopa therapy in the treatment of Parkinsons disease. The bioavailability of orally administered entacapone is, however, relatively low (29–46%). In this study we have prepared more lipophilic acyl and acyloxyacyl esters, an acyloxy alkyl ether and an alkyloxycarbonyl ester of entacapone, and we have evaluated them as potential prodrugs to enhance the oral bioavailability of entacapone. All the derivatives fulfilled prodrug criteria and released entacapone in human serum in‐vitro. The oral bioavailability of monopivaloyl (1a) and dipivaloyl (1b) esters of entacapone were investigated further in rats. The lipophilicity of 1b was high (log Papp 4.0 at pH 7.4) but its oral bioavailability was low (F = 0.6%), most probably due to its low aqueous solubility. The monopivaloyl ester of entacapone (1a) had a higher lipophilicity (log Papp 0.80) than entacapone (log Papp 0.18) at pH 7.4 while maintaining an aqueous solubility equal to entacapone. However, oral bioavailability was not increased when compared with the parent drug entacapone (F = 7.0% and 10.4%, respectively).

Fadolmidine‐induced ocular hypotension in normotensive rabbits

Fadolmidine, a novel selective α‐adrenoceptor agonist, was evaluated for its efficacy to lower intraocular pressure in normotensive rabbits (n = 5–6). The dose‐response profile between 0.004 μg and 12.5 μg of fadolmidine was determined. The effect of pH on the partition of fadolmidine was studied in order to select an optimal pH for topical fadolmidine administration. After topical administration, fadolmidine significantly lowered the intraocular pressure in normotensive rabbits. The onset of action was immediate, with no initial increase in intraocular pressure. A significant decrease in intraocular pressure was already observed at 1 h after dosing. The maximum decrease in intraocular pressure was observed after a 2.5 μg dose of fadolmidine in both eyes at 2 h after dosing. The mean maximum decrease in the treated and untreated eye was 6.4 mmHg and 3.9 mmHg, respectively. In conclusion, fadolmidine is a potent intraocular pressure lowering agent. In addition, fadolmidine does not cause a significant initial increase in intraocular pressure. Because of the strong dependence of the distribution coefficient on pH, the pH of the administered solution is important, with physiological pH being optimal in this respect.

2(S)-Amino-3-[1H-imidazol-4(5)-yl]propyl cyclohexylmethyl ether dihydrochloride and 2(S)-amino-3-[1H-imidazol-4(5)-yl]propyl 4-bromobenzyl ether dihydrochloride.

(Cyclohexylmethyloxymethyl)(1H-imidazol-4-iomethyl)-(S)-ammonium dichloride, C(13)H(25)N(3)O(+).2Cl(-), and (4-bromobenzyl)(1H-imidazol-4-iomethyl)-(S)-ammonium dichloride, C(13)H(18)BrN(3)O(+).2Cl(-), are model compounds with different biological activities for evaluation of the histamine H3-receptor activation mechanism. Both title compounds occur in almost similar extended conformations.