A Stability-Indicating RP-HPLC-UV Method for Determination and Chemical Hydrolysis Study of a Novel Naproxen Prodrug

A new naproxen amide prodrug was synthesized and spectrally characterized and a simple, precise, and accurate stabilityindicating RP-HPLCmethod was developed and validated for determination and chemical hydrolysis study of the prodrug. Forced degradation studies were conducted as per the International Conference on Harmonization (ICH) guidelines to establish the stability-indicating power of the method. Separations were performed on a C18 column (150 × 4.6mm i.d., 5μm p.s.). The mobile phase consisted of acetonitrile and phosphate buffer pH 4.0 in the ratio 60 : 40.The flow rate and injection volume were 1.0mL/min and 15 μL, respectively. The peaks were monitored at 272 nm. The average retention time is 5.136min. The linearity of the method was investigated in the range of 10–50 μg/mL and r was found to be larger than 0.9987. The LOD and LOQ were found to be 1.853 and 5.615 μg/mL, respectively. Results indicated that the degradants are well resolved and separated from the prodrug. Hydrolysis kinetics studies were carried out in buffer solutions (pH 1.2, 5.5 and 7.4) to establish the fate of the prodrug. The half-lives in the respective buffers were 23.5, 262, and 334 hours indicating sufficient stability to attain the goal of oral delivery.


Introduction
A prodrug is a modified biologically active compound that will release the active compound in vivo following enzymatic or chemical cleavage.Many drugs have unwanted properties that hinder their clinical use, such as susceptibility to acid hydrolysis in the stomach, causing damage to the stomach itself.There are many approaches to overcome those problems such as coating, different formulations, and different routes of administration.However, chemical derivatization of the drug into a prodrug remains the most common approach [1,2].Nonsteroidal anti-inflammatory drugs (NSAIDs) are the most commonly used agents for the symptomatic relief of fever, minor inflammations, headaches, swelling, sports related pains, and arthritis pain.They are also very helpful in postoperative, menstrual, and dental pains.Individually, they have different characteristics, but all of them share one pharmacological activity in-common, which is their ability to inhibit the cyclooxygenase (COX) enzyme family.
COX enzymes have many isoforms, but only two are well studied.They are COX-1 and COX-2.COX-1 is responsible for prostaglandins generation, which have cytoprotective effects on the stomach.COX-2 is mainly activated by inflammation.Therefore, most of the anti-inflammatory, antipyretic, and analgesic effects of the NSAIDs are attributed to their inhibition of COX-2, while most of their side effects can be related to inhibition of COX-1.Because NSAIDs are almost always used chronically especially by the elderly, their dangerous gastrointestinal (GI), cardiac, and renal side effects constitute a problem hindering their use.The prodrug approach is an effective way to mask the carboxylic acid group of NSAIDs, which is the main culprit in the GI adverse effects [3,4].
Naproxen is a NSAID of the phenylpropanoic acid class just like ibuprofen.Its GI toxic side effects are intermediate between ibuprofen (low) and indomethacin (high).Its structure is enantiomeric, which is (+)-2-(6-methoxy-2-naphthyl)-propanoic acid (Figure 1) [5].Naproxen (NP) still has all the side effects of NSAIDs; therefore, naproxen prodrugs were studied.Ester and amide prodrugs are the most studied prodrugs of naproxen.Studies show that they do reduce the GI adverse reactions of naproxen significantly [2,[5][6][7].
Quite a lot of different naproxen prodrugs were synthesized and reported in the literature [5,6,[8][9][10][11][12][13][14][15].However, the naproxen prodrug understudy here (referred to here after as pro-NP, Figure 2) contains and will release glycine in vivo which is not only safe but useful as a nutrient for the body.Furthermore, the intermediate of the synthesis of pro-NP is reported in the literature [16] to possess pharmacological activities.Microwave-assisted transesterification was used here for the first time to synthesize a naproxen prodrug.This reduces the transesterification time significantly and enhances the lipophilicity of the resulting prodrug which is crucial for oral activity.
Stability-indicating methods were described in the literature to study the chemical hydrolysis of naproxen prodrugs [12,13,15,[17][18][19].Most of them employed HPLC with UV detection and C 18 columns for its simplicity the same as in the method below.However, Rautio et al. [13] used a fluorescence detector and Sheha et al. [15] used a C 8 column.
In light of the above and in continuation of our interest to synthesize a GI sparing NSAIDs prodrugs [2,20], a new amide prodrug of naproxen (pro-NP) was synthesized and spectrally characterized and a validated stability-indicating RP-HPLC method was developed for studying the chemical stability of pro-NP.

Synthesis.
Pro-NP was synthesized by coupling of Snaproxen with glycine ethyl ester hydrochloride using DCC as coupling agent according to the literature method [2] (Scheme 1).The resulting intermediate was further subjected to microwave-assisted transesterification reaction in propanol using potassium cyanide as catalyst for five minutes.The final product was purified by column chromatography using hexane : ethyl acetate as mobile phase.

Chromatographic Conditions.
The HPLC method was developed on a Shimadzu prominence UFLC system (Shimadzu corporation, Kyoto, Japan) that consisted of a UV/Vis detector (SPD-20AV), a degasser (DGU-20A3R), column oven (CTO-20A), and a pump (LC-20AB).The column used was C 18 (150 × 4.6 mm internal diameter, 5 m particle size) at ambient temperature.Isocratic elution was used with the mobile phase consisting of acetonitrile and phosphate buffer pH 4.0 in the ratio 60 : 40.The flow rate and injection volume were 1.0 mL/min and 15 L, respectively.The peaks were monitored at the wavelength of 272 nm.The average retention time is 5.136 min.

Preparation of Standard Stock Solution.
A standard stock solution of pro-NP (100 g/mL) was prepared by dissolving accurately weighed 10 mg of the drug in 100 mL methanol.

Preparation of Phosphate
Buffer pH 4.0.6.80 g of KH 2 PO 4 was dissolved in 700 mL of water.Then the pH was adjusted when necessary with 10% v/v H 3 PO 4 .Water was added to complete to 1000 mL (BP 2013).

Forced Degradation Stability
Studies/Specificity. Forced degradation stability study was conducted as per ICH guidelines Q1A (R2) [21] and Q1B [22], regarding hydrolytic degradation, thermal degradation, photolytic degradation, and oxidative degradation.Results for forced degradation study are shown in Table 2.

Acidic and Alkaline Hydrolytic Degradation Study.
For the acid hydrolysis study, 2 mL of the pro-NP stock solution was transferred to a 20 mL volumetric flask and was completed up to the mark with 0.1 N HCl.A blank solution was also prepared.A determination of both solutions was made at zero time and after 24 h.For the base hydrolysis study, 2 mL separately of the pro-NP stock solution was transferred to two 20 mL volumetric flasks and completed up to the mark with 0.1 N and 0.01 N NaOH.Two blank solutions were made.
A determination of all four solutions was made at only zero time.
2.6.2.Oxidative Degradation Study. 2 mL of the pro-NP stock solution was transferred to a 20 mL volumetric flask and made up to the mark with 3% H 2 O 2 and an aliquot was injected into the HPLC at zero time and after 24 h.In another flask, 2 mL of the pro-NP stock solution was made to the mark with 6% H 2 O 2 and put in an oven set at 50 ∘ C for 2 h and then an aliquot was injected into the HPLC.Blanks were made and analyzed.

Thermal Degradation Study.
The solid pro-NP drug was put in an oven set at 70 ∘ C and an appropriate amount was taken, dissolved in methanol, and injected into the HPLC after 24 h and another after 48 h.

Photolytic Degradation Study.
The solid pro-NP drug was subjected to different lamps that emit visible light, short UV, and long UV.After two days, an appropriate amount was taken, dissolved in methanol, and injected into the HPLC.

Method Validation.
The proposed method was validated as per ICH guidelines [23] with respect to linearity, specificity, accuracy, precision, LOD, LOQ, and robustness.

Specificity (Selectivity).
In addition to forced degradation studies specificity was also confirmed by spiking pro-NP with naproxen (NP), which is a known degradant of pro-NP.10 mg of naproxen RS (NP) was dissolved in 100 mL of methanol to give 100 g/mL stock solution of NP. 2 mL of the NP stock solution was mixed with 4 mL of pro-NP stock solution and completed to 20 mL with methanol and an aliquot of the mixture was injected into the HPLC.The resolution (  ) between the two peaks was calculated.

Linearity.
The standard solutions for the estimation of linearity were prepared as per Table 1.Standard solutions used in the estimation of linearity (calibration curve) were prepared by suitable dilutions from the pro-NP stock solution to attain solutions of different concentrations.A plot of the peak area versus the concentration of the standard solutions was made (Figure 7).Regression analysis was carried out on the plot.From the regression data, the correlation coefficient ( 2 ), the slope of the line, the -intercept, and line equation were calculated in Table 3.

Precision.
Precision has been established in three steps: assessment of repeatability, interday precision assessment, and intraday precision assessment.

Robustness.
Small but deliberate changes were made to the mobile phase pH, which is 4.0, and the detection wavelength, which is 272 nm.Twelve determinations were made as follows: 3 at 270 nm, 3 at 274 nm, 3 at pH 4.3, and 3 at pH 3.7.The % RSD was calculated for the determinations at the changed parameters with the determinations at the normal parameter.

LOD and LOQ.
The LOD and LOQ were calculated from the equations where  is the standard deviation of the responses and  is the slope of the calibration curve.

Determination of the Hydrolysis Reaction Kinetics In Vitro.
Chemical hydrolysis was studied at 37 ∘ C ± 1 ∘ C in pH 1.2 (SGF), pH 5.5 phosphate solution (duodenal pH), and pH 7.4.Each reaction was initiated by taking 3 mL of the stock methanolic solution of pro-NP, to 20 mL volumetric flask, one drop of Tween-80 was added, and the volume completed to the mark with the buffer solution of the intended pH.The solutions were transferred to screw-capped test tubes.Samples were taken at appropriate time intervals spread over eight hours and chromatographed and the residual concentrations were calculated.

Results of Forced Degradation Stability Studies/Specificity.
During the forced degradation studies, the peaks of pro-NP were found to be pure and no significant degradation of drug substance was observed in thermal, photolytic, and oxidative conditions (Table 2).During the acidic hydrolysis analysis in 0.1 N HCl and after 24 h, a 21.5% degradation occurred (Figure 3).The guidelines [21,22] consider that amount of degradation is enough to be representative of all possible acidic hydrolysis degradation.The two possible acidic degradation products are (2-(6-methoxynaphthalen-2-yl) propanoyl)glycine (  = 2.4 min) and naproxen (  = 3.6 min) (illustrated in Figure 5).Moreover, during the alkaline hydrolysis analysis in both 0.1 N and 0.01 N NaOH at zero time, complete degradation of the drug substance occurred (Figure 4).This can only be explained by the breakdown of the ester bond in pro-NP and the formation of a soluble sodium salt.Furthermore, alkaline hydrolysis of esters is irreversible.Step 1

Specificity (Selectivity).
NP is an expected degradation product of pro-NP.NP seems to be the peak that elutes the closest to the drug peak; therefore, a mixture of NP : pro-NP in the ratio of 1 : 2 was prepared and injected into the HPLC.Through visual inspection alone, the observer can determine that those two peaks are well resolved;   > 1.5 (Figure 6).3.4.2.Linearity.The linearity of the method was determined at six concentration levels.A plot of peak area () against concentration (, g/mL) was drawn for pro-NP standard solutions with 10, 15, 20, 25, 30, and 50 g/mL concentrations.
The results revealed an excellent correlation between the peak area and analyte concentration, as evident by the coefficient of determination,  2 , which is greater than 0.9987.The calibration curve and its results are presented in Figure 7 and Table 3, respectively.

Precision.
The first step in establishing precision was repeatability.The % RSD of repeatability was 1.266.The % RSD of the intraday precision evaluation was 1.203, while that of interday precision was 1.673.The % RSD values of the repeatability, intraday, and interday studies were all <2, confirming that the method is precise.Calculations of the precision data are presented in Table 4.

3.4.4.
Robustness.In all the deliberately varied chromatographic conditions (mobile phase pH and the detection wavelength), the developed method showed satisfactory resolution (% RSD < 2), illustrating the robustness of the method (Table 6).
3.4.5.Accuracy.The accuracy was accepted based on the results of the linearity, precision, and sensitivity studies.This approach is accepted and justified by the ICH guidelines [23].
3.4.6.System Suitability.System suitability data are calculated in Table 5.   8).The rate constants ( obs ) and the corresponding half-lives ( 1/2 ) for the prodrug were calculated (Tables 7 and 8).The half-life was calculated using the equation  1/2 = 0.693/, which derives from the first-order kinetic law.In SGF, more than 20% of pro-NP was hydrolyzed after 8 h.In pH 7.4, only 2% of the drug was hydrolyzed after 8 h.In pH 5.5, the drug was found intact after 8 h, indicating complete absence of hydrolysis at that pH.The pro-NP was found to be stable in all investigated GI fluids with half-lives more than 20 hours which confirm that this prodrug will pass intact through GIT; when reaching the systemic circulation, a reasonable degradation will take place by nonspecific esterases and amidases in the liver to liberate the parent drug NP.

Conclusion
The method developed is simple, accurate, precise, specific, and robust.Forced degradation and spiking with degradants showed that the degradants are well resolved and separated from the prodrug.Pro-NP is therefore suitable to attain the goals of oral administration since it is stable during its passage through GIT.

Figure 3 :
Figure 3: Chromatograms indicating acid hydrolysis degradation of pro-NP.(a) is the chromatogram of pro-NP in 0.1 N HCl at zero time.(b) is the chromatogram of pro-NP in 0.1 N HCl after 24 h.(c) is the chromatogram of a blank in the same study.

Figure 4 :
Figure 4: Chromatograms showing the complete degradation of pro-NP in (a) 0.1 N NaOH and (b) 0.01 N NaOH.

Figure 5 :
Figure 5: The degradation pathway of pro-NP.

Figure 6 : 8 Figure 7 :
Figure 6: A chromatogram indicating the resolution between pro-NP and NP.

Table 1 :
Preparation of standard solutions for the evaluation of linearity.

Table 2 :
Results of forced degradation stability studies.

Table 3 :
Data from the regression analysis.

Table 4 :
Data from the precision evaluation.
3.5.Results of the Hydrolysis Study.The hydrolysis kinetics studies were carried out in aqueous buffer solutions (pH 1.2, 5.5, and 7.4) to establish the fate of the synthesized pro-NP.Under the experimental conditions, the targeted compound hydrolyzed to release the parent drug (NP) as evident by HPLC analysis.At constant pH and temperature, the reaction exhibited first-order kinetics, since plots of log concentration versus time resulted in straight lines (Figure

Table 7 :
The results from the hydrolysis kinetics.
*A is for pro-NP.

Table 8 :
The calculated rate constants and half-lives.