Identification of degradation products of saquinavir mesylate by LC-MS: Molecular docking and in silico ADME prediction studies

Saquinavir mesylate (SQM) is subjected to forced degradation under conditions of hydrolysis, oxidation, dry heat, photolysis as recommended by International conference on Harmonization guideline Q1A (R2). In total, (I-V) degradation products (DPs) were formed in acidic hydrolytic, alkaline hydrolytic and oxidative conditions. Successful separation of SQM and its DPs was achieved on C18(4.6mm×75mm) 3.5μg column at ambient temperature (30 ̊C) with mobile phase A (10mM ammonium acetate in water), B100% acetonitrile at 2.0ml/min flow rate in the gradient mode. The injection volume was fixed at 20μl and detection wavelength at 238nm. The HPLC method was found to be linear, accurate, precise, sensitive, specific, rugged, and robust for quantification of SQM as well as degradation products. The major degradation products (DP-1) formed in hydrolytic acid conditions was identified and characterized by LC-MS/MS and proposed the fragmentation patterns by comparing with SQM. Further, DP-1 were isolated through column chromatography and analyzed by 1H NMR. In Silico molecular docking studies on HIV protease (PDB: 4qgi) for DPs and SQM was estimated and found to be pharmacologically inactive than SQM. Prediction of Toxicity and ADME properities were performed for DP-1 and SQM and found to less toxic.


Introduction
Saquinavir mesylate is used to treat HIV infection by selectively binding to the protease enzymes and thus preventing its replication (Deeks, Smith, Holodniy, & Kahn, 1997). It is commercially marketed as Invirase in antiviral therapy of HIV-1, HCV infected patients (Geronikaki, Eleftheriou, & Poroikov, 2016).
It is the first drug to be available to HIV patients in United states as approved by USFDA in 2002(Kim, Dintaman, Waddell, & Silverman, 1998. SQM a peptidomimetic HIV protease inhibitor and has been effective in reducing viral load and mortality and is substrate for multidrug resistance transporter P-glycoprotein (P-gp) (Roberts, 1995).
ICH and FDA have provided guidelines for forced degradation studies for the investigation of degradation products of drugs and related substances (Procedures, 2000).
• A few HPLC methods reported for simultaneous estimation and identification of degraded products of SQM by LC-MS/MS. (Thummar et al., 2017).
• Recently Gananadhamu et al., has reported on forced degradation products of SQM by UPLC-ESI-Q-TOF-MS/MS where major degradation is achieved with acid hydrolysis (Mohit et al., 2017).
• The current study was to develop stability-indicated assay method for SQM, to identify, isolate and characterize the degraded product produced during the stability studies of SQM using HPLC-UV method. The SQM and major degradation product in acid hydrolysis (DP-1) were also carried out for molecular docking and In silico toxicity studies. Sample was optimally analyzed on a X-Bridge C 18 (4.6mm×75mm) 3.5µg column at ambient temperature (30˚C) with mobile phase A (10mM ammonium acetate in water), B100% ACN flowing at a rate of 2.0ml/min in the gradient mode. The injection volume was fixed at 20µl and detection wavelength at 238nm. The acid and alkali degraded drug solutions were neutralized and then diluted up to 10 times. The LC-MS studies were carried out using +APCI, ESI and modes of ionization with drug heated temperature of 180ºC; 10L/min, capillary voltage of 4.8kv, end plate off set voltage of 65V. Nebulizing (40 psi) gas. All spectra were recorded under identical experimental conditions in the positive ESI mode and with an average of 20 scans.

Forced degradation study
According to ICH guidelines Q1A(R2)(Guideline, 2012) Acid/Base degradation study: SQM was subjected to forced degradation study under acidic conditions by refluxing with 25ml of 1N HCl, and under basic conditions with 0.5ml of NaOH at 75˚C for 3hrs respectively.
Oxidative stress study was accomplished using 3% H 2 O 2 for 15 days.

Thermal degradation study
Drug was placed in a thermally controlled oven at 75˚C up to 72hrs.

Photolytic degradation study
Thin layer of drug solution was exposed to UV light of 320nm (200 watt-hour per square meter) for 72hrs and was kept at a distance of about 23 cm from the light source for 14 days with an exposure of 1.2 million lx h, for photolytic drug degradation.

Method validation
The method was validated in unison with ICH (International Conference on Harmonization) guideline Q2 (R1) for SQM and degradation product for linearity, accuracy, precision, and specificity.
Linearity linear for quantification of SQM and its acid degradation product in the concentration range of 5 to 30 µg/ml respectively.   Figure 2A) shows a complete degradation, whereas separation of base hydrolysis degradation products is satisfactory ( Figure 2B) Only 3.9% of SQM was observed by oxidative hydrolysis in presence of hydrogen peroxide. By photolytic and thermal degradation partial amount of SQM have been degraded. (Figures 3A-C). No formation of major degradation products has observed by oxidative, photolytic and thermal degradation.

Degradation Product Identification:
SQM was subjected to acid degradation using 1M HCl. After refluxing with 1M HCl, a complete degradation was observed. This reaction was controlled by RP-HPLC where complete fading of SQM peak was observed indicating complete degradation. It was also observed that one peak appeared at different time indicating the presence of only one degradation product. Structure elucidation of this degradation product was done by using 1 H NMR and mass spectral data. The LC chromatogram of isolated DP-1 is shown to be 99% purity (Fig 4). The mass spectrum of [M + H] + ions (m/z 553) of SQM acid degradation product (Fig 6)   Molecular docking, ADMET and Toxicity studies * Saquinavir mesylate and its major acid degradation product (DP-1) were subjected on HIV Protease (PDB ID: 4qgi) which is having a co-crystal saquinavir. The docking was done using FlexX module in LeadIT 2.1 software. The crystal ligand of SQM was redocked and calculated the binding affinity.    Conclusion 20 A validated forced degradation study was established to study the degradation product of SQM under acid, base hydrolysis, oxidation, photolysis and thermal stress conditions. The major acid degradation product (DP-1) were identified, isolated and characterized by 1 H NMR and Mass spectra data. In Silico molecular docking studies have revealed that DP-1 has shown weak interactions than SQM on HIV protease. Toxicity were assessed by using Osiris software and the results shown DP-1 has high irritant effect compared with SQM.