The present work was carried out to formulate and evaluate immediate release tablets of Inosine Acedoben Dimepranol (IAD). Tablets were prepared by wet granulation process to overcome the poor compression properties of IAD powder and produce high-dose tablets. Various type of binders and disintegrants like Povidone K-30, K-25, wheat starch, mannitol were used to prepare some series of tablets. Granules were evaluated for pre-compression parameters and tablets were evaluated for post- compression parameters. The composition batch № IAD L03 was the best for producing IAD 500 mg tablets. The optimal lubricant is glycerol dibehenate at a concentration of 3.08%. Optimized formulation was evaluated for in-vitro dissolution test. Stability studies were performed for the selected composition.

Inosine Acedoben Dimepranol, tablets, binders, dissolution

The active substance Inosine Acedoben Dimepranol (INN) (IAD), also known as Inosine pranobex and Methisoprinol, is an immunomodulator indicated for the treatment of viral infections such as influenza /viral respiratory infections, herpes simplex and herpes zoster, viral hepatitis, infectious diseases (rubella, chickenpox, measles, influenza) and others according to the summary of product characteristics (SmPC) Isoprinosine 500 mg tablets (Sliva et al. 2019).

The drug was developed by Newport Pharmaceuticals in the 1970s and distributed in more than 80 countries under the trademarks Isoprinosine, Inmunovir and Viruxan (owned by Newport). The active substance is a compound (complex) of inosine (1,9-dihydro-9-Dribofurasonyl-6H-purin-6-one) and a salt of 4-acetamidobenzoic acid with N, N dimethylamino-2-propanol in a molar ratio of 1: 3. It is a crystalline powder with a white to cream color and a characteristic odor, freely soluble in water, sparingly soluble in methanol, acetone, ethanol (Gordon 1974).

The active ingredient is not included in any pharmacopoeia.

When administered orally in men, Isoprinosine is rapidly and completely absorbed (≥ 90%) from the gastrointestinal tract and appears in the blood. In human subjects following a 1 g oral dose of Isoprinosine, the following plasma levels were found for DIP [N,N-dimethylamino-2-propanol] and PAcBA [p-acetamidobenzoic acid], respectively: 3.7μg/ml (2 hours) and 9.4μg/ml (1 hour) (Chen et al. 2013).

Maximum plasma inosine concentrations of the order of 0.6 mg /L were established within 1 hour after an oral dose of 1.5 g in healthy volunteers. Plasma concentrations cannot be determined approximately 2 hours after dosing. Plasma concentrations of inosine in healthy volunteers were maintained in the range of 100–1000 ng /ml after therapeutic doses, with no increase in correlation with dose increase.

There are no data for classification of IAD according to the Biopharmaceutics Classification System (BCS).

There are no published articles on the composition and production process of IAD tablets with immediate release of the active substance.

Patent ITMI932019 describes only the composition of IAD tablets without process: mannitol, starch, polyvinylpyrrolidone, magnesium stearate (Garzia Stefano and Brugola Luciano (1993).

According to the SmPC Isoprinosine 500 mg tablets, the composition is as follows: mannitol, wheat starch, povidone, magnesium stearate.

The aim of this work is to develop a stable product with fixed properties in the form of oral tablets, containing Inosine Acedoben Dimepranol 500 mg as active substance and with immediate release of the active substance.

The method for wet granulation is chosen, characterized by the ability to formulate high-dose tablets, as it is the case with the developed product. Other reasons are:

• Bulk density increase, which is due to multiple increase in the size of dust particles, which gives them excellent flow properties.
• The binding properties of the powders are improved, whereby they acquire the property of cohesiveness, which is especially important for obtaining quality tablets.

The povidone binder is included in powder form in the granulation mixture and granulated in situ by adding the solvent - aqueous alcoholic mixture (Bühler V 2005). This method is preferable in order to optimize the required amount of binder in the granules and to avoid exceeding the saturation limit of the granules (overwetting), especially in the case where the active substance is well soluble in water, such as IAD.

The results of the tested physicochemical parameters of the batches are presented in Tables 5, 6.

With regard to the „flow” property of the granules, the „Hausner ratio“ data showed all batches have good criterion values ​​except batches № IAD L01 and № IAD L02. The Hausner ratio may be related to the compressibility of powder and values < 1.25 are indicative for good compressibility (Wells 2005). In case of batch № IAD L01, the lowest value of the achieved bulk density of the granules is observed. This characteristic also correlates with the low hardness of the tablets.

As the amount of Povidone K25 binder (batches L01 ÷ L04) increased, the obtained granules showed higher density and better tableting behavior - high hardness, low friability and good disintegration.

The optimal properties of the granules and tablets are achieved in batch № IAD L03. Replacement of povidone type K25 with povidone type K30 (batch № IAD L05) results in granules with higher mechanical strength but with a longer disintegration time.

The achieved optimal lubrication system is from the substance Glycerol dibehenate - batch № IAD L03, at a concentration of 3.08%.

Decrease in the concentration of Glycerol dibehenate - batch № IAD L08 or the use of magnesium stearate - batch № IAD L06 or its combination with stearic acid - batch № IAD L07 indicates the appearance of defects in the tablets - breakage, capping the top of the tablet as a result of insufficient lubrication of punches and dies.

The disintegration time is not significantly affected in the different batches as the content of the disintegrant wheat starch that is the same in all batches.

According to the Guideline on the investigation of bioequivalence, the pH dissolution profile of the active substance must be determined. The active substance is considered to be very soluble if the highest dose administered as an immediate release form is completely dissolved in 250 ml of buffers with a pH range of 1 to 6.8 at 37 ± 1 °C.

The solubility of Inosine acedoben dimepranol in the study media is shown in Table 7:

The data show that the substance IAD is practically insoluble in acidic media. However, it meets the criterion D: S < 250 ml in media with a pH range of 4.5 ÷ 6.8 at a temperature of 37 °C.

According to the pharmacopoeia, a volume of solvent medium of 900 ml is normally specified for tablets. At a maximum dose of 1000 mg IAD, the concentration of IAD in the solution will be 1.1 mg /ml, which is many times lower than the concentration of saturated IAD solution. Therefore, in media with a pH range of 4.5 ÷ 6.8 at a temperature of 37 °C, the „Sink“ conditions will be met.

Experiments were performed for dissolution profile with batch № IAD L03 under the following conditions: pH 1.2, 4.5, 6.8 and stirrer speeds 50 rpm and 75 rpm.

The results of the dissolution profiles of the tested tablets are presented in the following graphs (Figs 1, 2).

Figure 1. 

Comparative dissolution profile of IAD 500 mg tablets, batch № IAD L03, in medium with pH 1.2, 4.5, 6.8 (Ph Eur) and stirrer speed 50 rpm.

The following observations were made during the test:

pH 1.2

• At stirrer speed 50 rpm

The product is practically insoluble, large relative standard deviations.

pH 4.5

• At stirrer speed 50 rpm

Dissolution is delayed, large relative standard deviations.

Figure 2. 

Comparative dissolution profile of IAD 500 mg tablets, batch № IAD L03, in medium with pH 6.8 (Ph Eur) and stirrer speed 50 rpm and 75 rpm.

There is a “coning” effect.

pH 6.8

• At stirrer speed 50 rpm,

Dissolution is delayed, large relative standard deviations.

There is a “coning” effect.

• At stirrer speed 75 rpm.

Complete dissolution, small relative standard deviations - less than 10%.

According to the test conditions, 75 rpm and phosphate buffer pH 6.8 are the most suitable stirrer speed and dissolution media for IAD 500 mg tablets.

In addition, a dissolution profile test was performed under the established conditions - pH 6.8 and stirrer speed 75 rpm and the market product Isoprinosine 500 mg tablets (batch № 1980065, expiration date 02/2024), in order to assess the similarity of the profile to that of IAD 500 mg tablets, batch № IAD L03.

The results are presented in Fig. 3.

Figure 3. 

Comparative dissolution profile of Isoprinosine 500 mg tablets, batch № 1980065 and IAD 500 mg tablets, batch № IAD L03.

The dissolution profiles of both products show similar behavior and are characterized as similarly rapid - 85% of the active substance is released within 30 minutes.

To demonstrate the similarity of dissolution profiles according to the Guideline on the investigation of bioequivalence, the value of the similarity factor f₂ is calculated:

f_{2 inosine} = 72.59 and

f_{2 4- acetamidobenzoic acid salt} = 74.34

According to criterion f₂, values between 50 and 100 indicate the similarity of the dissolution profiles of the two products.

The discriminant power of the dissolution method was tested in an experimental batch in which the binder povidone was added twice as high as an amount. In this case, it is assumed that the binding forces (cohesion) in the tablet will be increased, which will lead to a delay in the disintegration time and respectively a delay in the dissolution time. This behavior will be reflected in the specified dissolution rate parameter.

The data obtained from the study are the following:

• The dissolution rate for 30 minutes of Inosine is 74% and of 4-acetamido benzoic acid salt is 79%. The obtained values do not meet the specified norms - 85% of the active substance is released within 30 minutes.
• The process of dissolution of the tablet is slow due to the increase in the amount of binder.
• The chosen dissolution method allows discrimination of batches with different in vitro release characteristics.

Batch № IAD L03 500 mg tablets was packed in a blister consisting of PVC /PVdC film (250 μm /40 g /m²), thermally sealed with aluminum foil (20 μm) and stability tests were performed under accelerated conditions - 40 °C /75% RH for six months according to Note for guidance on stability testing.

The analytical results are presented in Table 8:

The results of the batch impurity profile after 6 months of storage under accelerated conditions did not show any significant change.

No changes in the physicochemical parameters of the tablets were observed.

The results correspond to the product specification.

Based on the results of the study, an optimal composition and process for the production of IAD 500 mg tablets with immediate release of the active substance is proposed.

The tablets are prepared by wet granulation process with povidone 25, which is included in the inner part of the granules.

The study shows that the substance IAD is practically insoluble in acidic media. However, it meets the criterion D: S < 250 ml in media with a pH range of 4.5 ÷ 6.8 at a temperature of 37 °C.

A method for testing the dissolution of tablets has been developed and the following conditions have been established: apparatus, stirrer speed and type of medium.

A liquid chromatographic method for determining the content of IAD has been developed.

The similarity of the active substance release profiles between the test product – IAD 500 mg tablets and the reference product – Isoprinosine 500 mg tablets has been demonstrated.

The results of the stability study indicate that the tablets remain stable under the conditions of observation.