Chromatography Forum

Hello,

I must analyze a drug who contain the latanaprost molecule. I have find a method with mobile phase acetonitrile/water 35/65 with octane sulfonate 10nM adjusted pH3.5 with hydrochloric acid. This molecule contains an ester function who hydrolyse in water. I think the pH used at 3.5 keep acid function, but I don't understand why octane sulfonate. You can find the molecular formula at the following adress :

http://dailymed.nlm.nih.gov/dailymed/fd ... pe=display

Thank you for help

For this molecule, you do not need acid nor an ion-pair reagent. Maybe the ion-pair reagent was added to influence the retention of other analytes???

I have read the publication. Others molecules are: alcohol, water, ester fatty acid. The latanaprost is instable at room temperature in water (hydrolyse of ester function) but not at 5°C or in oil. That's all I know.

That ester can not be that unstable as it is presented in an aqueous solution according to your link. There might be a confusion with the acid or base (fairly strong) catalysis of the hydrolysis. The FA esters also shouldn´t responf to that HCl or the ion pairing agent.

Abstract :

"Latanoprost in water is not stable against heat stress due to hydrolysis of the isopropyl ester in the latanoprost molecule. Therefore, the storage condition of latanoprost ophthalmic solution, Xalatan® brand, was in a low temperature (2–8 °C). We formulated a favorable ophthalmic lipid emulsion of latanoprost using polyvinyl alcohol as emulsifier which showed a good heat stability. The assays of the latanoprost ophthalmic lipid emulsions adjusted to pH 5.0, 6.0 and 7.0 were 100.4%, 100.7% and 99.2% after storage for 4 weeks at 60 °C, respectively. The possibility of room temperature storage for the latanoprost ophthalmic lipid emulsion was demonstrated."

Yusuke Sakai, , Shin-Ichi Yasueda and Akira Ohtori
Senju Pharmaceutical Co. Ltd., 1-5-4 Murotani, Nishi-ku, Kobe, Hyogo 651-2241, Japan
Received 30 June 2005; revised 11 August 2005; accepted 31 August 2005. Available online 3 October 2005.

I would take note that Latanopost is a prodrug that is activated by the hydrolysis of the ester. In clinical application, a small amount of hydrolysis occuring during storage over several weeks could be significant in the delivery of the drug - and therefore likely the reason for the discussion of hydrolysis.

The hydroysis on an HPLC column during a half hour chromatographic run should be insignificant, as it is with other esters.

So it is much more stable than I had guessed.

My comment is only based on examination of the structure in the link above. In the region of the ester, there is nothing to activate the the ester to make it any more suseptible to hydroslysis than any other ester at the end of an alkyl chain.

At the same time, it is completely baffling to me why anybody would want to use an ion-pair reagent and a low pH for the chromatography of a neutral drug that is less stable at acidic pH, which would decompose to an entity of the same charge as the ion-pair reagent....

youp

My idea is certainly crazy but maybe they want to transform the ester in acide. At the same time they create a reaction between alcohol functions and chlorhydric acid. Then they use octane sulfonate who combine with carbocation.

Don_Hilton, in regard to stability I was referring to the 60° test mentioned by lotusx06.
lotusx06, have you had a look at conditions for the reaction of alcohols with acids, especially primary alcs? There is nothing here about which to be crazy.
This appears simply to be another case of where we are not sufficiently informed or some people with extraordinary skills have been at work.

I am analyst not organist, but I have search informations. In theory, esterification is possible but I'm not sure immediatly at room temperature. This reaction is slow, accelerate with increase of temperature, is not complete and reversible.
The presence of strong acid (I don't find conditions of pH and concentration of acid), catalysis hydrolysis of the ester. Maybe strong acid catalysis also a sort reaction of esterification with alcohol function to form a carbocation, then ion reagent can intervene.

No "maybe" here as far as I (physicakl organic chemist) can see.

(Incidentally, we were talking about hydrolysis, not esterification. With acid catalysis this can be reversible, but not in the presence of such a large excess of water).

Have you tried using the method without octane sulfonate? With or without a buffer?