Synthesis and characterization of impurities of an anti-psychotic drug substance, Olanzapine

HPLC analysis of olanzapine 1 , a known anti-psychotic drug


Introduction
Olanzapine 1, an atypical anti-psychotic drug 1,2 with a thienobenzodiazepinyl structure, is indicated for the treatment of schizophrenia.It displays a broad pharmacological profile and is a selective monoaminergic antagonist with high affinity binding to serotonin 5HT 2A/2C , dopamine D 14 , muscrinic M 1-5 and adrenergic α 1 receptors 3,4,5 .
The HPLC analysis of olanzapine displayed six impurity peaks in the range of 0.05 to 0.15% levels along with the olanzapine peak.As per the guidelines recommended by ICH, the acceptable level for a known or unknown related compound (impurity) is less than 0.15 and 0.10 % respectively in a drug substance.In order to meet the stringent regulatory requirements, the impurities present in the drug substance must be identified and characterized.Our present work deals with the identification, synthesis and characterization of impurities/related substances of olanzapine 1.

Results and Discussion
Olanzapine was synthesized in the laboratory by a known pathway (Scheme 1) as per the literature method 6 which involved the reaction of 2-methyl-4H-3-thia-4,9-diazabenzo[f]azulen-10-ylamine hydrochloride 2 with piperazine to give 3, which on further methylation with dimethyl sulfate yielded olanzapine 1.A typical analytical LC chromatogram of a laboratory sample of olanzapine displayed impurities/related compounds over a range of 0.05-0.15%.These impurities were identified, synthesized and characterized by spectral analysis.The assigned structures of these impurities are namely 2-methyl-4,9-dihydro-3-thia-4,9-diaza-benzo Impurity 4, a potential impurity formed during the synthesis of olanzapine due to basic reaction conditions, was prepared by reacting compound 2 with sodium hydroxide (Scheme 2).

Scheme 2
Acetic acid is used in the work-up of the olanzapine synthesis.Thus compound 3 was Nacetylated resulting in the formation of impurity 5.This impurity was synthesized quantitatively by the reaction of compound 3 with acetic anhydride (Scheme 3).

Scheme 3
Any residual 2 present in the reaction reacts with compound 3 to give impurity 6.This impurity is formed only in trace quantities (Scheme 4).Methylation of compound 3 with dimethyl sulfate forms olanzapine 1 (Scheme 1) however, as a side reaction, the free NH group present in the diazabenzoazulene moiety of 1 is methylated, resulting in impurity 7.This impurity was synthesized by the reaction of olanzapine with excess dimethyl sulfate (Scheme 5).

Scheme 5
Compound 8, is formed by the reaction of olanzapine 1 with dichloromethane, a solvent that is used for the final purification of olanzapine.This impurity was synthesized by prolonged contact of olanzapine with dichloromethane at elevated temperatures and its structure was further confirmed by its spectral data (Scheme 6).Impurity 9 is formed in traces due to aerial oxidation of olanzapine 1.This impurity was synthesized by the oxidation of olanzapine 1 with m-chloroperbenzoic acid and was purified by column chromatography (Scheme 7).Experimental Section General Procedures. 1 H NMR spectra were recorded on a Gemini 200 MHz FT NMR spectrometer; the chemical shifts are reported in δ ppm relative to TMS.The IR spectra were recorded in the solid state as KBr dispersion using Perkin Elmer FT-IR Spectrophotometer.The mass spectra were recorded on Shimadzu LCMS-QP8000, LC-MS and AB-4000 Q-trap LC-MS/MS.