Synthesis, Characterization, Identification and Quantification of Sertraline Hydrochloride Related Impurities

Sertraline hydrochloride is an antidepressant of the selective serotonin reuptake inhibitor (SSRI) class, is synthesized for commercial use as a drug substance in highly pure form. During the synthetic process development studies of Sertraline hydrochloride, an impurity was detected in the final product at levels ranging from 0.05% to 0.15% in Gas Chromatography (GC) method and its molecular weight was determined by LC-MS analysis. The impurity was identified as 4-(4-chlorophenyl)-3,4-dihydronaphthalen-1(2H)-ol impurity, synthesized and characterized, the process of its formation was discussed in detail. After qualification of impurity standard, it was used as a reference standard for development of quantification by High Performance Liquid Chromatography (HPLC) method.

Sertraline (trade names Zoloft and others) is an antidepressant of the selective serotonin reuptake inhibitor (SSRI) class [1,2]. It was introduced to the market by Pfizer in 1991. Sertraline is primarily prescribed for major depressive disorder in adult outpatients as well as obsessive-compulsive disorder, panic disorder, and social anxiety disorder, in both adults and children [3][4][5]. In 2013, it was the most prescribed antidepressant and second most prescribed psychiatric medication (after alprazolam) on the US retail market, with over 41 million prescriptions [6].
Sertraline hydrochloride drug substance is official in United State pharmacopoeia (USP) [7] as well as European Pharmacopoeia (EP) [8]. The listed organic impurities and method of analysis by gas chromatography is same in both pharmacopoeias. In most of the commercial batches one unknown impurity is observed at relative retention time (RRT) 0.66 is about 0.05% to 0.15% ( Figure 2). Here in this research article attempts were made to synthesize, isolate, characterize, identify and quantify the unknown impurity of RRT 0.66.

Materials and Methods
Manufacturing process of sertraline unknown impurity at RRT 0.66

4.
After complete addition of aluminium chloride, start heating to raise the temperature of reaction mass to 60-65°C. 5. Maintain the temperature of the reaction mass to 60-65°C and check the progress of the reaction on TLC till complete conversion of α-Naphthol (Limit: 1.0%). If not, then maintain further the reaction mass to 60-65°C till α-Naphthol is in limit.

Characterization data
In GC purity of crude material batch No. SET/40/II/270/49 split peak was observed at RRT 0.66 so further study is carried out for separation and isolation of two peaks by HPLC. After few trials both peaks were separated by HPLC in batch No. SET/40/II/270/49. The first peak is eluting at retention time 27.7 minutes of purity 45.69% and second peak at 32.9 minutes of purity 54.19%. The method was developed suitable for Liquid Chromatography-Mass Spectrometry (LCMS) study and mass of both peaks are determined. The mass of both peaks is observed same.

LCMS spectra
Instrument: LCMSD_ALS LCMS chromatogram and spectrum is attached in Figure 3.
Molecular ion peak observed in LCMS at RT=13.02 min is 257.92. Molecular ion peak observed in LCMS at RT=14.3 min is 257.76 Further for isolation of both peaks of batch No. SET/40/II/270/49 preparative HPLC Technique was used. After isolating both peaks HPLC purity of individual isomer was checked and found Isomer-I eluting at retention time 24.5 minutes purity is 98.29% and peak purity on PDA detector is passing. Second isomer eluting at retention time 28.9 minutes purity is 99.18% and peak purity on PDA detector is passing.
Isomer-I and Isomer-II both are separately spiked in the sertraline hydrochloride sample and injected in GC purity method. It is found that the area of unknown impurity at RRT 0.66 has been increased in the sample spiked with Isomer-II. Isomer-II is exactly matching with the unknown impurity of RRT 0.66. While Isomer-I is eluting closely after Isomer-II i.e., at RRT 0.67. For further confirmation GCMS was carried out and found that fragmentation pattern of unknown impurity at RRT 0.66 and Isomer-II is same.
From the above study, it can be concluded that the unknown impurity at RRT 0.66 in GC purity is Isomer-II. For quantification of unknown impurity at RRT 0.66 that is Isomer-II by GC the low load of Isomer-II i.e., 0.1% with respect to test concentration was injected in GC but it was found that the pure Isomer-II is thermally unstable and after degradation one degradant peak is eluting at retention time 7.1 minutes.
From above study, it is decided that unknown impurity in GC of RRT 0.66 to be disregarded in GC purity method and it has to be quantified by following HPLC method.

Calculation
Integrate the peaks of Isomer I and Isomer II in standard and test solution and calculate both isomers by following formula.

Area of isomer I or isomer II peak in Test solution
Conc of specified isomer in std.solution mg / mL x P x 100 % specified impurity Avg. area of isomer I or isomer II peak in std. solution Where, P=Potency of specified isomer I or isomer II reference standard.

Chiral HPLC method of separation of enantiomers of isomer I and isomer II of 4-(4-Chlorophenyl)-3,4-Dihydronaphthalen-1(2h)-ol
Instrumentation and liquid chromatographic conditions: HPLC method was performed using a Shimadzu-2010 CHT HPLC system with UV detector and Lab Solution software. Separation was achieved with the mixture of Mobile Phase-n-Hexane: Isopropyl alcohol: 1-Propanol: Diethylamine (490:0.5:2.5:0.5 v/v/v/v) in isocratic elution. The column temperature was maintained at 25°C. Ultraviolet detection was performed at 275 nm. Injection volume is 20 µL and run time is 60.0 minutes. HPLC column is Chiralpak AD-H, 250 mm length, 4.6

System suitability criteria
The system is suitable for analysis, if and only if, a) Resolution between closely eluting Isomer-II peak-1 and Isomer-II peak-2 should be Not less than 1.5 in system suitability solution.

Results and Discussion
Organic impurities of Sertraline hydrochloride are determined by gas chromatography method as per USP and EP monograph. In most of the commercial batches one major unknown impurity was observed at RRT 0.66 consistently about 0.15%. This impurity is synthesized with help of chemical synthesis, by thin layer chromatography and by preparative HPLC.
During chemical synthesis, it had been observed that two impurities are formed which are eluting very closely in GC purity method at RRT 0.66 and 0.67. This chemically synthesized crude material with mixture of two impurities was applied on preparative HPLC and two separate pure impurities are isolated. The purity of isomer I is 98.29% and isomer II is 99.18%. Characterization of Isomer I and isomer II is done for structure elucidation with infra-red spectrometry (IR), NMR and mass.

Characterization of Isolated Isomer I
Accordingly, a first aspect of the present invention provides a compound Isomer I having chemical name (1S,4S)-4-(4-chlorophenyl)-
In further studies, it was observed that the isolated impurities in their pure form are getting degraded in GC purity method because both are thermally unstable. So, for quantification of both impurities i.e., isomer I and isomer II of 4-(4-chlorophenyl)-3,4-dihydronaphthalen-1(2H)-ol HPLC method is developed.
Further for separation of enantiomers of both isomers chiral purity method was developed. In chiral purity method isomer I eluted as two enantiomers with the retention time of Isomer-I peak-I is about 26.83 minutes and Isomer-I peak-2 is about 41.94 min while Isomer-II peak-I is about 31.24 min, Isomer-II peak-II is about 34.75 min [9][10][11][12].

Conclusion
In conclusion, a process related impurity of sertraline hydrochloride produced according to the synthetic route explained above. Two impurities were identified, synthesized and characterized. Structural elucidations of both synthesized compounds were done by using proton NMR, IR and MS spectral data. Thus, the regulatory requirement was fulfilled by characterizing this impurity and the prepared impurity standard was used during analytical method development studies. Hence, the reported route of synthesis can be used for isolation of impurity and the analytical methods can be used for routine determination of both impurities in sertraline hydrochloride in quality control laboratories in the pharmaceutical industry.