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  Bioequivalence Study and Pharmacokinetic Evaluation of Two Brands of Tadalafil 20 mg Tablets in an Indian Population


J.S.K.Nagarajan, V.J.Mohanraj, B.Suresh and A.B.Balaji

Bioequivalence and Quality Control Laboratory, J.S.S. College of Pharmacy, Ooty, India




Objective: To examine the in vivo average bioequivalence of a locally manufactured Indian brand Tadalafil and foreign brand, both in 20mg tablet form and to evaluate the drug’s pharmacokinetics in Indian male volunteers.


Design and Participants: Twelve healthy male adults completed the study. Inclusion and exclusion criteria are detailed in the paper. The protocol was approved by the Institute of ethical committee and the study was conducted according to the standards.  This was an open-label, randomized, single dose, two sequence, two way, cross over design, with days wash-out. The extraction procedure and HPLC analysis were adapted from previously published methods.


Results: The non-compartmental pharmacokinetic analysis was done using PK Software. The bioequivalence statistical model used is a two period crossover analysis of variance with the factors sequence, period, treatment, and subject-within-the sequence. The primary pharmacokinetic parameters compared were Cmax and AUC0-µ and were Ln transformed. The 90% CI was calculated for the parameters.


Conclusion: The non-transformed pharmacokinetic parameters (Cmax, Tmax, AUC0-12, AUC0-µ, t1/2) and the geometric least square means are calculated for both brands. The confidence intervals for Cmax and AUC0-µ are fully contained within the interval 80-125%, thus establishing bioequivalence.




Tadalafil is an oral drug that is used for treating impotence (the inability to attain or maintain a penile erection). It is in a class of drugs called phosphodiesterase inhibitors that also includes sildenafil and vardenafil. The pharmacokinetics of tadalafil is well-documented in the literature;1-5 after single oral-dose administration, the maximum observed plasma concentration (Cmax) of tadalafil is achieved between 2.05 to 2.13 hours. Absolute bioavailability of tadalafil following oral dosing has not been determined. The rate and extent of absorption of tadalafil are not influenced by food; thus tadalafil may be taken with or without food. From the literature it is found that the mean apparent volume of distribution following oral administration is approximately 63 L, indicating that tadalafil is distributed into tissues. At therapeutic concentrations, 94% of tadalafil in plasma is bound to proteins. The mean oral clearance for tadalafil is 2.5 L/hr and the mean terminal half-life is 16.76 hours in healthy subjects. Tadalafil is excreted predominantly as metabolites, mainly in the feces (approximately 61% of the dose) and to a lesser extent in the urine (approximately 36% of the dose).3,6

To date, there is no documented evaluation of tadalafil pharmacokinetics in the Indian population. Furthermore, the general pharmacokinetic characteristics of these populations are poorly known. This study was designed to evaluate the pharmacokinetics of tadalafil in Indian healthy male volunteers and to assess in vivo average bioequivalence of locally manufactured tadalafil (Product A) and imported brand (Product B). The 20mg tablet form was used for both brands.

The HPLC method adapted and used for tadalafil assay in serum is sensitive, simpler, and less time consuming.7 It has the advantage of using only 200mL of serum for determination of tadalafil concentration.



Twelve healthy Indian male adults completed the study. The demographics are shown in Table 1. All subjects are Indians, within –15% to +10 % of normal body weight (according to Broca’s formula), non-smokers, and consume alcohol occasionally or not at all.

Subjects were excluded if they had clinically or biologically significant abnormalities, history of allergy to tadalafil or to any of the excipients in the two brands, history of patients who take nitrates for angina, tadalafil could cause heart pain or possibly even a heart attack by exaggerating the increase in heart rate and the lowering of blood pressure. Tadalafil also exaggerates the blood pressure lowering effects of some alpha-blocking drugs e.g., terazosin (Hytrin) that primarily are used for treating high blood pressure or enlargement of the prostate. Individuals who take these alpha-blockers should also be excluded.

The protocol was approved by the Institute of ethical committee and the study was conducted in accordance with the guidelines on good clinical practice and with ethical standards for human experimentation. Every subject provided a written consent and was free to withdraw from the study at any time without any obligation.

Study Protocol

This was an open-label, randomized, single dose, two-way, non-replicated cross-over investigation, with a wash-out period of 10 days between the doses. Subjects were randomized to one of the tadalafil brands during phase I of the study. After a wash out period of 10 days, each subject received the other brand. Subjects participating in the study were not allowed to take any medicines 2 weeks prior to the study and until the completion of the second phase of the protocol.

After fasting overnight, the subjects reported to the study unit. Each subject took the tablet with 240 cc water and fasted for 3 hours post-dose.  All subjects ate a standardized breakfast, lunch and dinner at 3, 6 and 9 hours post-dose respectively.

Table 1. Age, Height and Weight of Selected Volunteers






Age (years)




Height (cm)




Weight (kg)





Blood (5 cc) was drawn, via an indwelling intravenous catheter for determination of tadalafil concentration, at 0, 0.5, 1, 1.5, 2, 3, 4, 6, 7, 8, 10, 12 and 24 hours post-dose. Blood was immediately centrifuged and serum was stored at –70°C until analysis (around 1 month later).


Table 2. Pharmacokinetic Parameters of Both Brands of Tadalafil


Local Brand Tadalafil

Foreign Brand Tadalafil

Percentage (%)

AUC0-24 (ng/mL)





















Table 3. Ln Transformed Data









90% Confidence Interval


93.37 – 107.10

95.01 – 105.23

The assay was performed using high-performance liquid chromatography (column Lichro Cart C18, 250 x 4.6 mm, 5m) with both PDA and UV detection at 280 nm and flow rate of 1.0 mL/min. The mobile phase consisted of acetonitrile and triethylamine in the ratio of 42: 58 at a pH of 3.5.

Tadalafil and internal standard stock solutions were prepared in acetonitrile and triethylamine were stored at –20°C in the dark (they remained stable for minimum 3 months). Working standards were prepared by diluting the stock in drug-free Plasma over various concentrations (20, 50, 100, 200, 400, and 800 ng/mL). 1ml of plasma sample (which was obtained from the study subjects) transferred into centrifuge tube and 0.5ml of 4ug/ml of Internal standard solution and 0.5ml of precipitating agent were added.  The sample was further vortexed for 5 minutes for an addition of 5 minutes before centrifugation at 4000rpm for 10 minutes.  The supernatant layer was separated and injected the same for analysis. The mean recovery of Tadalafil was around   96.0.%. The limit of detection was 2.0ng/mL and the standard curve showed perfect linearity up to 500ng/mL (which was the highest concentration tested).

Clinical examination and laboratory analysis were performed 1 week before the study and clinical examination was conducted after completion of the first phase and second phase. Adverse events were monitored throughout the study and kept track of in a special register. A physician was available throughout the study to examine and evaluate any adverse events and patient compliance.




Pharmacokinetic analysis using the untransformed data was performed using PK software and a non compartmental approach. The untransformed pharmacokinetic parameters for both brands are represented in Table 2 (the values less than the limit of detection of the method were not considered in the analysis). Log (natural) transformation of Cmax, AUC0-12 and AUC0-µ were performed prior to the statistical analysis.


The parametric general linear model used for statistical analysis includes factors accounting for sequence effect, subjects nested in sequences, period and treatment. The equivalence criteria range is 80 to 125% according to FDA guidelines.


The 90% confidence intervals for the ratio of geometric least square means were calculated for each parameter. The significance level was set to 0.05. Results are presented in Table 3. No sequence effect, period effect or treatment effect were detected.




We studied the pharmacokinetics of tadalafil in Indian healthy male adults and the results obtained are close to previously published tadalafil pharmacokinetic data. However, our data seem to indicate that the terminal half-life of the drug is more prolonged, with an average of 16.76 hours for both brands. A recent study has demonstrated that tadalafil is predominantly metabolized by CYP3A4 to a catechol metabolite. The catechol metabolite undergoes extensive methylation and glucuronidation to form the methylcatechol and methylcatechol glucuronide conjugate, respectively. The major circulating metabolite is the methylcatechol glucuronide. Methylcatechol concentrations are less than 10% of glucuronide concentrations. In vitro data suggests that metabolites are not expected to be pharmacologically active at observed metabolite concentrations.

As for the comparison of the two tadalafil brands, the 90% confidence intervals of Cmax, AUC0-12 and AUC0-µ are fully included within the 80 to 125% interval, thus establishing bioequivalence.





The locally manufactured brand of tadalafil 20mg tablet form is bioequivalent to the imported brand and both were well tolerated. No subject developed any adverse experience.

Further studies of the ethnic related specificities of tadalafil pharmacokinetics are needed, particularly concerning the metabolism of the drug as well as the relation between efficacy of tadalafil and its pharmacokinetics.


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