Department Of Pharmaceutics, Kokan Gyanpeeth Rahul Dharkar College Of Pharmacy & Research Institute, Karjat
Sustained-release (SR) coated tablets are formulated to release the active pharmaceutical ingredient (API) at a controlled rate, maintaining therapeutic plasma concentrations over a prolonged period. This strategy improves patient compliance, reduces the frequency of dosing, and limits fluctuations in drug levels. The formulation usually involves functional polymeric coatings that control drug release through mechanisms such as diffusion, erosion, or osmotic pressure. Key factors that influence the release profile include coating thickness, polymer type, plasticizer concentration, and manufacturing conditions. SR coated tablets provide important benefits in chronic therapy and remain a crucial technology in modern pharmaceutical development
OVERVIEW OF SRDDS: The main purpose of sustained-release (SR) dosage forms is to minimize side effects by delivering the drug at a controlled, steady rate, maintaining consistent therapeutic levels over a defined period. These systems aim to improve bioavailability and enhance the overall effectiveness of medications. They achieve this by targeting the drug to the site of action, reducing the frequency of dosing, and ensuring uniform drug release, which together lead to better therapeutic outcomes (1). Several types of sustained-release oral dosage forms have been developed, including osmotic systems, matrix systems made with water-soluble or insoluble polymers or waxes, and membrane-controlled systems. Recent research has focused on applying SR systems to drugs with poor water solubility. When designing such systems, two important factors to consider are the drug’s half-life and its pharmacological activity (2). Among the various sustained release systems, matrix tablets have gained significant attention due to their simplicity of formulation, reproducibility, stability, and suitability for industrial manufacturing. In matrix systems, the drug is uniformly dispersed within a polymeric matrix that controls drug release primarily through diffusion, erosion, or a combination of both mechanisms (56, 57). SR formulations are designed to release the drug gradually over an extended period, maintaining a stable blood concentration. This not only enhances patient compliance but also improves the overall effectiveness of the treatment (3). Several statistical methods are widely used to develop an optimized sustained-release tablet formulation with the desired dissolution profile, while reducing the number of experimental trials and saving time. Consequently, computer-aided optimization techniques, such as artificial neural networks (ANN) and response surface methodology (RSM) based on polynomial equations, have become increasingly popular in formulation development (4). The multilayered system is frequently used in sustained-release formulations, providing a mechanism that controls and extends drug release as it dissolves or diffuses (5). One of the earliest contributions to sustained drug delivery can be traced back to Israel Lipowski’s 1938 patent. This work involved coated pellets designed for prolonged drug release and is considered a precursor to the coated particle approach, which became more widely developed in the early 1950s (6).The primary objective of any drug delivery system is to deliver the right amount of drug to the intended site, achieving the desired therapeutic effect quickly and maintaining effective drug levels over time (7, 8). Sustained-release systems are drug delivery systems designed to release the drug slowly over an extended period. When such a system is able to maintain consistent drug levels in the blood or target tissue, it is classified as a controlled-release system. If it extends the duration of action compared to conventional formulations but does not maintain constant levels, it is termed a prolonged-release system (9). If the systems can provide some control, whether this is of temporal or spatial nature, or both, of drug release in the body, or in other words, the system is successful at maintaining constant drug levels in the target tissue or cells, it is considered a controlled-release system (10). According to pharmacokinetic principles, the ideal way to minimize the Cmax to Cmin ratio is through zero-order absorption, which results in a constant plasma drug concentration once steady state is reached, as long as absorption continues. Successfully bringing an extended-release formulation to market is challenging and requires careful consideration of factors such as the physicochemical properties of the drug, physiological influences, and manufacturing parameters (11, 12).
Figure 1: Plasma drug concentration profile
3. OBJECTIVES OF ORAL SUSTAINED RELEASE DOSAGE FORM
a. To keep the drug level in the body steady and effective over a specified duration.
b. To decrease how often the drug needs to be administered compared with conventional dosage forms.
c. To ensure the drug reaches the intended site of action, thereby reducing or avoiding unwanted side effects.
d. To enable targeted delivery, such as directing the drug to particular receptors, cells, tissues, or organs.
e. To enhance the safety of highly potent drugs by limiting unnecessary systemic exposure.
f. The occurrence of both local and systemic adverse effects can be minimized, particularly in sensitive patients (13-16).
4. SCOPE AND APPLICATIONS OF SUSTAINED RELEASE TABLETS (17-19)
Sustained release formulations are widely utilized in multiple therapeutic fields, such as:
a. Cardiovascular disorders: Helps in maintaining steady blood pressure control in patients with hypertension.
b. Diabetes management: Supports continuous regulation of blood glucose levels through extended drug activity.
c. Pain management: Providing continuous analgesic effects with reduced dosing.
d. Neurological disorders: Enhancing the effectiveness of drugs in Parkinson’s and Alzheimer’s diseases.
e. Antibiotic therapy: Maintains therapeutic drug concentrations for longer periods, enhancing antibacterial effectiveness and lowering the risk of resistance development.
5. MECHANISMS OF DRUG RELEASE (20, 21)
Sustained release formulations use various mechanisms to control drug release, including:
a. Diffusion-Controlled Release
The drug is embedded within a polymer matrix or enclosed by a polymer membrane, and its release takes place through diffusion across the polymer structure, as seen in matrix and reservoir delivery systems.
b. Erosion-Controlled Release
• The drug is loaded into biodegradable polymer systems that slowly break down, allowing controlled release of the drug over time, as observed in hydrogel-based delivery systems.
Figure 2: Mechanism of drug release
6. FACTORS AFFECTING DRUG RELEASE (22-26)
Several variables play an important role in determining the drug release rate and performance of sustained-release tablets, including:
A) Physicochemical Properties of the Drug:
a. Solubility: Drugs with low aqueous solubility often need specialized formulation approaches to achieve controlled release.
b. Partition coefficient: Influences the drug’s ability to diffuse through polymer matrices or membranes.
c. Drug stability: Instability or degradation within the gastrointestinal tract can alter release behavior and effectiveness.
B) Polymer Type and Concentration:
a. Hydrophilic polymers (such as HPMC) absorb water, swell, and form a gel layer that controls the rate of drug release.
b. Hydrophobic polymers (such as ethyl cellulose) function as diffusion barriers, thereby slowing drug release.
C) Tablet Size and Shape:
a. Tablet size: Larger tablets generally dissolve more slowly, which can extend the release duration.
b. Tablet shape: Non-uniform shapes may lead to uneven drug distribution and inconsistent release patterns.
D) Compression Force During Manufacturing:
a. High compression: Produces denser tablets, which can slow down drug diffusion.
b. Low compression: May lead to quicker disintegration, compromising sustained-release properties.
E) Influence of Food and GI Enzymes:
a. Food intake: Can modify the rate of drug release and absorption.
b. Gastrointestinal enzymes: May break down some drugs, necessitating protective, enzyme-resistant coatings.
7. EVOLUTION OF SUSTAINED RELEASE TABLETS (27)
The field of controlled drug delivery has advanced considerably over the years. Traditional tablets and capsules were initially designed for immediate drug release, which required patients to take multiple doses daily. To achieve prolonged and consistent drug action, researchers developed strategies to modulate drug release, such as:
a. Polymer-based matrix systems: Control drug diffusion through a polymer network.
b. Coated tablets: Enable delayed or extended drug release.
c. Osmotic pump systems: Provide zero-order release, delivering the drug at a constant rate.
d. Microencapsulation and nanotechnology: Allow precise control over drug release profiles.
Table 1: Polymers used in sustained release tablets (28, 29)
|
Polymer |
Type |
Role |
|
Ethyl cellulose |
Hydrophobic |
As a diffusion barrier |
|
Hydroxypropyl methyl cellulose (HPMC) |
Hydrophilic |
Forms a gel layer |
|
Polyvinyl alcohol |
Hydrophilic |
Provides controlled release |
|
Sodium alginate |
Natural polymer |
Provides controlled release |
|
Carbopol |
Hydrophilic
|
Swells to form a gel, controlling drug diffusion |
|
Xanthum gum |
Natural polymer |
Provides viscosity |
|
Chitosan |
Natural bioadhesive |
Enhances drug absorption |
|
HPMC pthalate |
Enteric polymer |
Provides pH sensitive coating |
Table 2: Excipients used in sustained release tablets (30-32)
|
Excipients |
Type |
Role |
|
Lactose |
Diluent |
Provides bulk |
|
Microcrystalline cellulose (MCC) |
Diluent |
Enhances tablet compressibility |
|
Starch |
Binder |
Improves tablet binding |
|
Magnesium stearate |
Lubricant |
Reduces friction |
|
Talc |
Glidant |
Improves tablet flow properties |
|
Xanthum gum |
Viscosity enhancer |
Modifies Viscosity |
|
Polyethylene glycol (PEG) |
Plasticizer |
Enhances flexibility |
|
Hydroxypropyl methyl cellulose (HPMC) |
Film forming agent |
Controls drug release |
|
Eudragit |
Coating polymer |
Provides enteric or sustained release coating |
|
Polyvinylpyrrolidone (PVP) |
Binder |
Improves tablet binding and adhesion |
8. STANDARD TEST FOR PARMACEUTICAL TABLET (33-35)
a. Description
This test focuses on the visible characteristics of a tablet. It includes observing features such as shape, color, coating, and any markings. For example, a tablet may appear white, round, biconvex, film-coated, and carry an imprint like “Rx” on one side. This evaluation is qualitative in nature and relies on visual inspection rather than measurement.
b. Identification
The purpose of this test is to confirm the presence of the correct active pharmaceutical ingredient (API) in the tablet. It is designed to distinguish the intended drug substance from other compounds, even those with similar chemical structures. This ensures that the product contains the right medicinal component (36)
c. Assay
Also known as the content determination test, this analysis measures the quantity of the API present in the tablet. It provides a numerical value that indicates whether the drug content falls within the specified limits, ensuring the correct dosage is delivered.
d. Impurities
This test evaluates the presence of unintended substances in the tablet, other than the API and excipients. These may include degradation products or by-products formed during manufacturing or storage. It is often used as a stability-indicating test to ensure the product remains safe and effective over time (37, 38)
9. IN PROCESS AND FINISHED PRODUCT QUALITY CONTROL TESTS FOR TABLETS
a. In-process quality control (IPQC) tests are carried out during manufacturing to ensure that tablet production remains consistent and within specified limits. These tests monitor various physical and process parameters such as temperature, compression pressure, moisture level, processing time, tablet weight, particle size distribution, hardness, loss on drying, disintegration time, color, and overall tablet integrity.
b. Finished product quality control (FPQC) tests are performed after manufacturing is complete to verify the final quality of the tablets. These include evaluation of assay (drug content), uniformity of content, variation in weight, friability, hardness, disintegration time, and dissolution behavior.
Standard pharmacopoeias recommend specific IPQC and FPQC tests to ensure that tablets meet required quality, safety, and efficacy standards.
c. Size and Shape
The dimensions and form of a tablet depend on the required dosage and intended use. These characteristics are controlled during the compression stage through the selection of appropriate tooling (dies and punches). Consistent monitoring ensures uniformity across all tablets in a batch.
d. Colour and Odour
Colour is often used to differentiate and identify tablets, as well as to improve patient acceptability. It must remain uniform within a batch and between different batches. Any noticeable change in odour, especially in products like vitamin tablets, may indicate degradation or stability problems. For chewable tablets, taste also plays an important role in patient acceptance.
e. Hardness Test
The hardness test is used to evaluate the strength of sustained-release (SR) tablets and their ability
Figure 4: Vernier Calliper
to withstand mechanical stress during handling, packaging, and transportation. This ensures that the tablets remain intact, preventing premature drug release or inaccurate dosing. Hardness is measured using devices such as the Monsanto tester, which applies force to the tablet until it fractures. The result is expressed in kilograms (kg) or Newtons (N). For SR tablets, a typical hardness range is 4–8 kg, which provides sufficient strength while still allowing proper dissolution. (39, 40)
f. Thickness and Diameter
Measuring the thickness and diameter of tablets is crucial to ensure uniform size, which is particularly important for sustained-release (SR) tablets. Any variation in size can result in inconsistent drug release and dosing. Consistent tablet dimensions help maintain uniform drug content and release profiles across the batch. Thickness is typically measured with a vernier caliper or micrometer, while diameter is assessed using a digital micrometer (41)
g. Weight Variation
The weight variation test evaluates the consistency of tablet weight within a batch. This is especially important for sustained-release (SR) tablets, as differences in weight can lead to variations in drug content and release behavior.
The weight variation test is carried out to ensure consistency in the weight of tablets within a batch. According to the USP, 20 tablets are selected and weighed individually. The average weight is calculated and each tablet’s weight is compared with this average.
Permissible Limits for Weight Variation (42-44)
|
Average Tablet Weight |
IP/BP Limits |
USP Limits |
|
80 mg or less |
±10% |
130 mg or less: ±10% |
|
80 mg to 250 mg |
±7.5% |
130 mg – 324 mg: ±7.5% |
|
250 mg or more |
±5% |
More than 324 mg: ±5% |
h. Friability Test
The friability test assesses how easily a tablet can chip, break, or crumble during handling, packaging, or transportation. This is particularly important for sustained-release (SR) tablets, as high friability can cause tablet breakage and compromise their controlled-release performance.
Figure 5: Roche Friabilator
In this test, tablets are placed in a rotating drum, commonly using a Roche Friabilator, and exposed to mechanical friction for a set number of rotations, usually 100 revolutions. Afterward, the tablets are weighed again, and the percentage of weight loss is calculated. For sustained-release (SR) tablets, a weight loss of less than 1% is considered acceptable, indicating that the tablets are durable enough to withstand handling without affecting drug release (45, 46)
i. Disintegration test
The disintegration test measures how quickly a tablet breaks down into smaller particles under specified conditions. The acceptable time limits vary depending on the type of tablet and the pharmacopoeial standards followed (IP, BP, USP).
Standard Disintegration Time Limits (42-43)
|
Type of Tablet |
IP Limit (min) |
BP Limit (min) |
USP Limit |
|
Uncoated tablets |
15 |
15 |
5–30 minutes |
|
Coated tablets |
60 |
60 |
1–2 hours |
|
Enteric-coated tablets |
60 |
— |
About 1 hour or as specified in the monograph |
|
Film-coated tablets |
30 |
— |
30 minutes or as specified |
|
Effervescent tablets |
5 |
5 |
Less than 3 minutes or as specified |
|
Soluble tablets |
3 |
3 |
— |
|
Dispersible tablets |
3 |
3 |
Less than 3 minutes or as specified |
|
Orodispersible tablets |
— |
3 |
— |
|
Gastro-resistant tablets |
— |
60 |
— |
j. Dissolution Testing
Dissolution testing evaluates the rate at which the active pharmaceutical ingredient (API) is released from an SR tablet. This test is crucial because it determines whether the formulation can deliver the drug in a controlled and sustained manner, ensuring the intended therapeutic effect. Dissolution studies are typically performed using USP (United States Pharmacopeia) dissolution apparatus, which comes in two main types:
• USP Apparatus I (Basket Method)
A mesh basket is used to hold the tablet, which is then immersed in a rotating vessel.
Figure 6: USP Apparatus I
•USP Apparatus II (Paddle Method)
In this method, the tablet is placed in a dissolution medium and stirred with a rotating paddle. This apparatus is often preferred for sustained-release (SR) tablets because it provides better control over the dissolution environment. For SR formulations, the dissolution profile should demonstrate a consistent and gradual drug release over an extended period (typically 12–24 hours), avoiding any rapid or “burst” release at the beginning. The ideal release pattern is steady and linear.
Release Kinetics and Mathematical Modeling
To understand the mechanism of drug release more precisely, the data from in-vitro dissolution studies are analyzed using mathematical models. The release profile is fitted to various kinetic models, such as zero-order, first-order, Higuchi, or Korsmeyer-Peppas, to determine how the drug is released from the SR formulation.
Figure 7: USP Apparatus II
• Zero-order kinetics: The drug is released at a constant rate over time, which is ideal for sustained-release (SR) tablets.
• First-order kinetics: The release rate depends on the remaining drug amount in the tablet, which occurs in some SR formulations.
• Higuchi model: Describes drug release from diffusion-controlled matrix systems, commonly used for matrix-based SR tablets.
• Peppas model: A combined model applicable when both diffusion and polymer erosion
• influence drug release.(47)
Using these models, formulators can understand the release mechanism and optimize tablet design to achieve the desired drug release profile.
Acceptance Criteria for Dissolution Test of Extended/Prolonged-Release Tablets (USP, BP, IP) (41, 42)
The dissolution test for extended or prolonged-release tablets is performed in multiple stages to ensure consistent and controlled drug release over time.
Stage-wise Acceptance Criteria
Stage L1
Number of tablets tested: 6
Criteria:
• Every individual tablet must fall within the specified release limits at each time point.
• Each tablet must meet at least the minimum required drug release at the final sampling time
Stage L2 (if L1 criteria are not fully met)
Additional tablets tested: 6 (total = 12) Criteria:
• The average of all 12 tablets must lie within the specified limits
• The average must not be below the required amount at the final time point
• No individual tablet should deviate by more than ±10% of the labeled drug content from the specified limits
• No tablet should be more than 10% below the required drug release at the final time
Stage L3 (if L2 criteria are still not satisfied)
Additional tablets tested: 12 (total = 24)
Criteria:
• The average of all 24 tablets must remain within the specified limits
• The average must not fall below the required final release value.
• Not more than 2 tablets may deviate by more than ±10% of labeled content from the limits.
• Not more than 2 tablets may be more than 10% below the required final release.
• No tablet should deviate by more than ±20% of labeled content from the limits.
• No tablet should be more than 20% below the required final release
k. Stability Testing
Stability testing ensures that SR tablets retain their quality, efficacy, and safety throughout their shelf life. These studies examine changes in the tablet’s physical characteristics, chemical composition, and drug release behavior under different environmental conditions (48)
a. Accelerated Stability Studies
In accelerated stability testing, tablets are stored under high temperature and humidity (e.g., 40°C and 75% relative humidity) for a specific period, typically 6 months. This simulates long-term storage and helps predict shelf life. Tablets are periodically evaluated for appearance, hardness, dissolution rate, and drug content to monitor stability over time (49)
b. Long-Term Stability Studies
Long-term stability studies are carried out under recommended storage conditions (e.g., 25°C and 60% relative humidity) over an extended period, typically 12 months or longer. These studies provide detailed information on the drug’s stability over time, ensuring that the sustained-release (SR) tablet retains its therapeutic effectiveness without degradation. Parameters such as dissolution, drug content, and physical properties (appearance, hardness, and friability) are monitored regularly (50)
l. Moisture Content
Moisture content is a critical factor influencing the stability of SR tablets, especially those with hydrophilic matrices. Excess moisture can lead to premature drug release or API degradation. Moisture is measured using methods like Karl Fischer titration or loss on drying (LOD), which quantify the water content in the tablets. Maintaining a low moisture level is ideal to prevent degradation and ensure consistent controlled release.
m. Drug Content Uniformity (42-44)
The content uniformity test is performed to ensure that each tablet contains a consistent amount of the active pharmaceutical ingredient (API). Typically, 10 tablets are selected randomly and analyzed individually.
Pharmacopoeial Criteria
This test is generally required when the API content is below certain limits:
• IP (Indian Pharmacopoeia): less than 10 mg or less than 10% of tablet weight
• BP (British Pharmacopoeia): less than 2 mg or less than 2%
• USP (United States Pharmacopeia): less than 25 mg or less than 25%
Acceptance Criteria
Stage 1 (10 tablets):
• Not more than 1 tablet may fall outside the range of 85% to 115% of the average content
• No tablet should fall outside 75% to 125% of the average content
• The relative standard deviation (RSD) should be ≤ 6%
Stage 2 (if required):
• If 2 or 3 tablets fall outside the 85%–115% range (but none outside 75%–125%), an additional 20 tablets are tested
Final Evaluation (30 tablets total):
• The batch complies if no more than 3 tablets are outside the 85%–115% range
• None of the tablets should be outside the 75%–125% range
10. IN-VIVO EVALUATION
In-vivo studies are performed to verify that the in-vitro drug release data correspond to actual therapeutic outcomes in the body. These evaluations are important to confirm the clinical effectiveness of sustained-release (SR) tablets.
a. Bioavailability Studies
Bioavailability is the proportion of the administered drug that reaches the systemic circulation in an active form. These studies are essential to ensure that the SR tablet delivers the drug effectively over the intended duration. Key pharmacokinetic parameters measured include:
• Cmax: Maximum plasma concentration
• Tmax: Time to reach maximum concentration
• AUC: Area under the plasma concentration–time curve
For SR tablets, the objective is to maintain plasma drug levels within the therapeutic range for a prolonged period while avoiding high peak concentrations that could cause toxicity.
b. Bioequivalence Studies
Bioequivalence studies compare the pharmacokinetic profile of the SR formulation with a reference product, which could be an immediate-release formulation or another marketed SR product. These studies confirm that the new SR tablet provides similar therapeutic benefits as the reference formulation. (52, 53)
11. Microbiological Evaluation (For Antibiotic SR Tablets)
For SR tablets containing antibiotics or other antimicrobial agents, microbiological testing is essential to confirm the sustained antibacterial activity. This test assesses whether the SR tablet maintains its efficacy against target microorganisms over the duration of its release. Techniques such as minimum inhibitory concentration (MIC) determination and zone of inhibition tests are commonly used to evaluate the antimicrobial potency of the SR formulation (54,55)
CONCLUSION
The development of sustained-release tablets continues to be a rapidly evolving area of pharmaceutical research, with innovations aimed at optimizing drug delivery and therapeutic outcomes. Modern approaches focus on advanced release mechanisms that allow precise, controlled drug delivery over extended periods. These include environment-responsive polymers (sensitive to pH or temperature) and technologies such as osmotic pumps, which provide constant drug release. The rise of personalized medicine—where treatment is tailored to an individual’s genetic profile and disease condition—is also driving the development of SR formulations that better meet specific patient needs. Emerging technologies like 3D printing are enabling the creation of complex tablet structures with precisely controlled release profiles, enhancing both efficacy and safety.
Despite these advancements, several challenges remain. Designing SR formulations that achieve consistent, controlled drug release requires careful selection of excipients, optimization of polymer systems, and precise control of manufacturing processes. Variability in the gastrointestinal environment—such as changes in pH, motility, and enzyme activity—can affect drug release and absorption, making it difficult to achieve predictable therapeutic effects. Ensuring drug stability during prolonged release and preventing interactions with excipients are also critical concerns. Additionally, regulatory requirements for SR tablets are stringent, often requiring extensive clinical studies to demonstrate safety, efficacy, and consistent release. Manufacturing costs are typically higher due to specialized excipients, complex production methods, and rigorous quality control, which can limit affordability, especially in low-income regions.
Nevertheless, emerging technologies and innovations offer promising solutions. Advances in drug delivery systems, formulation methods, and manufacturing techniques can improve the quality, accessibility, and efficacy of SR tablets. The use of natural polymers, biodegradable materials, and patient-specific formulations is likely to make future SR tablets more efficient and patient-friendly. Through ongoing research, collaboration, and technological development, SR tablets have the potential to provide a targeted, sustainable, and effective approach to drug therapy, meeting the evolving demands of modern medicine.
REFERENCES
Siddhi Sawant, Sanket Dharashivkar, Nilesh Bonde, A Comprehensive Review On: Sustained Release Tablets, Int. J. of Pharm. Sci., 2026, Vol 4, Issue 4, 2890-2903, https://doi.org/10.5281/zenodo.19641029
10.5281/zenodo.19641029
