multi-layer tablets
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MULTILAYER TABLET: A NEW TREND IN SOLID DOSAGE FORMS
Nikhil T. Gavate1*, Shital B. Gondkar1, Ravindra B. Saudagar2
1Dept. of Pharmaceutics, R.G. Sapkal College of Pharmacy, Kalyani Hills, Anjaneri, Nashik-
422213. (INDIA) 2Dept. of Pharmaceutical Chemistry, R.G. Sapkal College of Pharmacy, Kalyani Hills,
Anjaneri, Nashik-422213. (INDIA)
ABSTRACT
Pharmaceutical products that are designed for oral delivery
are currently available mostly in the immediate-release type. These are
designed for immediate release of drug and rapid absorption. For
added advantages of therapy and enhanced efficacy sustained and
controlled release formulations are being used more and more. These
forms also offer the advantage of patient compliance. Several
advantages over the conventional are seen but still some problems
arises in preparation of this kind of dosage form such as physical
incompatibility, chemical incompatibility etc. Therefore the bilayer
and multilayer tablets are known as a novel drug delivery system.
Multilayered tablets possess various benefits, namely the ability to
prevent incompatibility between drugs and Excipients; and by
providing multiple release kinetics profiles in single delivery system of either the same or
different drugs, by means of different release control mechanisms. The aim of this article is to
review multilayered tablet dosage forms and outline its advantages. Various factors
governing the design and evaluation of these types of tablets have also been discussed.
Keywords: Sustained drug delivery, Controlled drug delivery, multilayered tablets, Novel
drug delivery system.
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Article Received on 17 October 2013, Revised on 23 November 2013, Accepted on 20 December 2013
*Correspondence for
Author:
Nikhil T. Gavate
Dept. of Pharmaceutics, R.G.
Sapkal College of Pharmacy,
Kalyani Hills, Anjaneri,
Nashik-422213. (INDIA),
gavatenikhil@gmail.com
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INTRODUCTION
Oral drug delivery has been known for decades as the most widely utilized route of
administration among all the routes that have been explored for the systemic delivery of
drugs via various pharmaceutical products of different dosage forms. The reasons that the
oral route achieved such popularity may be in part attributed to its ease of administration as
well as the traditional belief that by oral administration the drug is as well absorbed as the
foodstuffs that are ingested daily. In fact, the development of a pharmaceutical product for
oral delivery, irrespective of its physical form (solid, semisolid, or liquid dosage form),
involves varying extents of optimization of dosage form characteristics within the inherent
constraints of gastrointestinal (GI) physiology. [1]
Problems related with conventional drug delivery system are:
Poor patient compliance.
Improved chances of missing the dose of a drug with short half-life for which repeated
administration is necessary.
The unavoidable fluctuations of drug concentration may lead to under medication or over
medication.
A typical peak-valley plasma concentration time profile is obtained which makes
attainment of steady-state condition difficult.
The fluctuations in drug levels may lead to precipitation of adverse effects especially of a
drug with small Therapeutic Index whenever over medication occur. [2]
However, patient compliance is likely to be poor when patients need to take their
medication three to four times daily on chronic basis. Thus, these shortcomings have been
circumvented with the introduction of controlled release dosage forms.
Pharmaceutical products designed for oral delivery and currently available on the prescription
and over-the-counter markets are mostly the immediate-release type, which are designed for
immediate release of drug rapid absorption. Because of their clinical advantages over
immediate-release pharmaceutical products containing the same drugs, sustained-release
pharmaceutical products, such as those formulated on the basis of spansule coating
technology, have over the past decade gradually gained medical acceptance and popularity
since their introduction into the marketplace. Recently, a new generation of pharmaceutical
products, called controlled-release drug delivery systems, such as those developed from the
osmotic pressure-activated drug delivery system, have recently received regulatory approval
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for marketing, and their pharmaceutical superiority and clinical benefits over the sustained-
release pharmaceutical products have been increasingly recognized.[2] Through such dosage
forms several advantages over the conventional are seen but still some problems arises in
preparation of this kind of dosage form such as physical incompatibility, chemical
incompatibility etc. Therefore the bilayer and multilayer tablets are known as a novel drug
delivery system.
Zero order sustain release
It comprises either a hydrophilic or hydrophobic intermediate layer containing the active drug
or one or two barrier layers which are press coated to the faces of the tablet core, leaving the
sides of the core exposed. Many authors have evaluated this design. [3] The widely used
barrier polymers for sustaining the drug delivery are either hydrophilic and/or hydrophobic
materials. In general linear release profiles can be obtained by applying hydrophilic barrier
layers on both the faces of a hydrophobic matrix tablet or by applying a hydrophilic barrier
layer on one face and hydrophobic barrier layer on the other face of the matrix tablet.
However, net formulation and variables within the matrix and barrier layers is important to be
controlled rather carefully in order to achieve zero-order drug release from hydrophobic
matrix tablet coated with hydrophobic barrier layers on both the faces.
Time Programmed Delivery System
Time programmed followed by time controlled release, when the delivery of drug is required
in a time controlled fashion in the gut, rather than release of drug in continuous
manneraccording to circadian rhythm. This system consists ofcore which is coated with
different polymeric barriers. The release of drug from the core tablet after swelling or after
erodingof hydrophobic or hydrophilic barrier of coating thatshow pulsatile release of the drug
followed by extended or prolonged release of the drugdelivery system provides immediate
release of the drug. [4, 5]
Bimodal Release Profile
Bimodal release profile show an initial rapid release followed by slow release and again a
second phase of rapid drug release i.e. sigmoidal release profile. This system compensates the
slow absorption in the stomach and small intestine and for programmed pulse releases that
perform more effectively at the site of action to undertake periodic changes. [6, 7]
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Quick/Slow Delivery System
Quick/slow delivery system which is characterised by initial rapid release achieves
therapeutic effect immediately sustains a constant release of drug to maintain plasma level
concentration. This concept applied in cases where dose regimen does not satisfy simple
release of the drug. [8]
MULTILAYER TABLET
There are many reasons for the development of two or more drugs incorporated in single
dosage form in several diseases. This concept not only use in case of emergencies but also in
common disease like Parkinson, allergic condition etc. [9] The intension behind development
of such formulation is that when patient unable to take frequent dosing due to various
reasons. [10]
This concept was preferred for combination of different APIs in single dose therefore to get
relief in single dose and to attain site therapeutic concentration immediately and also
maintain same dose over 12hr. [11] FDCs gaining importance globally which are
therapeutically justified and are available in critical disease condition e.g. AIDS, Cancer and
Pulmonary disease. Multilayer tablet technology contains two or more APIs being used in
treatment of wide range of chronic condition. It is applicable for broad range of compatible as
well as incompatible drug for both immediate and sustained release dosage forms. Currently
about two-third of all prescriptions are dispensed as solid dosage forms. [12]
Many combinations are available and being used which can provide immediate and modified
release of two drugs or dual release rate of same drug in single dosage form by using
hydrophilic and hydrophobic polymer matrices. It is therapeutically justified that combination
of modified and immediate release matrix found to increase bioavailability. [13] Compaction
of different granules in the form of various layer in single tablets are called as multilayer
tablets. It generally consists of parallel, clear, coloured, visual distinct layers two to three or
more APIs or APIs along with functional or non-functional placebo layers, sometimes to
avoid interaction between different incompatible layers. Oral solid multiple drug delivery
system in the form of FDCs are beneficial than the other oral formulation.
Three layer tablets are formulated for controlled release, usually consists of drug core layer
sandwiched by external layers, which may contains different amount of drug to form a
concentration gradient matrix or just act as a barrier layer in order to restrict release or to
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minimize burst effect upon in-vivo placement. [14] In the multilayer tablet one swellable layer
and one erodible layer whose functions are to control the hydration and swelling rate of the
core, and thereby slow down dissolution of the drug. [15] When the tablet comes into contact
with gastric juices, it increases considerably in volume and thus remains in the stomach for a
longer time.
Multilayered systems which contains bilayered, triple-layered, quadruple-layered, etc are
becoming increasingly recognized as controlled-release drug delivery systems. Multilayered
tablets possess various benefits, namely the ability to prevent incompatibility between drugs
and excipient; and by providing multiple release kinetics profiles in single delivery system of
either the same or different drugs, by means of different release control mechanisms,
immediate drug release using a disintegrating monolithic matrix in order to achieve an initial
peak in plasma drug level, delayed drug release reachieved using an eroding monolithic
matrix which may deliver another active drug to the latter part of gastrointestinal tract,
controlled release swelling monolithic matrix carry out together swell as well as erode in
which drug constantly released and better management control and regulation of release
profiles by retarding initial burst release and achieving zero-order kinetics. [16]
Multi-layer tablet dosage forms are designed for a variety of reasons [17]
1. To separate incompatible Active pharmaceutical ingredient (APIs) from each other, to
control the release of API from one layer by utilizing the functional property of the other
layer.
2. To control the delivery rate of either single or two different active pharmaceutical
ingredient.
3. To administer fixed dose combinations of different APIs, prolong the drug product life
cycle, fabricate novel drug delivery systems such as chewing device, buccal/
Mucoadhesive delivery systems, and floating tablets for gastro-retentive drug delivery.
4. To modify the total surface area available for API layer either by sandwiching with one or
two inactive layers in order to achieve swellable /erodible barriers for modified release.
Objective of preparing multilayer tablets [18-20]
1. To use different APIs in combination having proven advantages over single compounds
administered separately for therapeutic effect.
2. To overcome the limitations in case of a single drug which is unable to treat or avoid
adverse drug effect, if any.
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3. To get dual release profile so as to reduce dosing frequency and thereby increasing patient
compliance.
4. To combine compatible or incompatible drugs with different release characteristic in
same dosage form and enhancing the stability of dosage form as compared to its
conventional dosage form.
5. To treat critical disease condition when single active unable to produce complete
therapeutic action and to maintain over a period 12 h or more.
Ideal properties of Multilayer tablets [21]
Drug should not be affected by compaction of each layer and physically stable; and
withstand the mechanical shock.
Separation of layers should not occur during various stages such as compression, coating,
packing, shipping and storage.
Layer should not fuse into non-disintegrating matrix, should have clear, parallel, visual
separation in final compressed tablets.
If it consists of disintegrating matrix, it should be disintegrated within GIT, modified
release part should not be affected dissolution profile of IR part and slow and gradual
erosion of second layer for slow release of drug.
TYPES OF MULTILAYER TABLETS
Bilayer tablets to quadruple layered tablets are available.
1. BILAYER TABLET
Bilayer tablets are suitable for sequential and simultaneous release of two different API’s. In
this one layer is immediate release and another layer is sustained release act as a maintenance
dose. Bilayer tablet is suitable mean of to deliver two drugs at one time without any dynamic
and pharmacological interaction.
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2. TRIPLE LAYER TABLET
Triple layer tablet consist of three layer of which first layer is for immediate release of drug
and the second layer is for sustained release. These two layersare separated with the middle
barrier layer. This is more suitable for the delivery of two drugs which have interactions in
them.
3. TABLET IN TABLET
4. SURROUNDUNG COATED CORA TABLET
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Multilayer Tablet and Controlled Release
Multilayer tablet consists of layers of drug with different release rate, having ability to
prevent drug-excipient incompatibility. It provides multiple release kinetics profile in single
delivery system of one or more drugs. In this immediate release and then sustained release of
drug is designed as control system. Immediate release layer is designed with disintegrating
monolithic matrix in order to achieve initial peak and sustained release layer is designed with
erodible monolithic matrix to deliver the drug as later part to maintain the drug plasma
concentration.
Design of Multi-Layered Tablets[22 - 24]
The design of multi-layer through modulating layers which allows different tablet designs
for the production with specific release to achieve different dissolution patterns like bimodal,
delayed and multi modal delivery have been given below:
Zero order sustained release
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Time programmed delivery system
Bimodal release profile
Quick / slow delivery system
Generally the drug release mechanism from hydrophilic, swellable matrices is a coupling of
polymer macromolecular relaxation and drug diffusion. Both the phenomena depend initially
on the rate at which water may enter the device. Multi layered design is based on the
following aspects:
A. Matrix hydration rate and consequent swelling and/or lowering of diffusion rate;
B. Modulation of the surface of matrix through which the drug can be delivered.
Major advantages of bimodal matrix tablets include [25]
Change in release rate upon medium change.
Applicability to different types of drugs
Complete dissolution in human body fluids avoiding empty remnants; and
The possibility to achieve therapeutic blood levels similar to those obtained by
administration of two smaller doses over an extended period of time.
Advantages of the multi-layer tablet dosage form [26]
Lighter and compact.
Flexible Concept.
Cost is lower compared to all other oral dosage form.
They are unit dosage form and offer the greatest capabilities of all oral dosage form for
the greatest dose precision and the least content variability.
Easy to swallowing with least tendency for hang-up.
Objectionable odour and bitter taste can be masked by coating technique.
Suitable for large scale production.
Greatest chemical and microbial stability over all oral dosage form.
Easiest and cheapest to package and strip.
Disadvantages of Multilayer Tablet Dosage Form [27]
Difficult to swallow in case of children and unconscious patients.
Some drugs resist compression into dense compacts, owing to amorphous nature, low
density character.
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Problems during In-process Quality Control (IPQC) such as layer separation during
storage, packaging, shipping, insufficient hardness, cross-contamination etc.
Regulatory aspects that it should be based on believable therapeutic and medical
justification which is clinically relevant.
Set up of bilayer or trilayer tablets is time consuming, take long time to initial setup,
speed, and many times problem arise during tableting as compared to single layer
conventional tablet press in machine setup, cost and production output.
Parameters to be considered during Multilayer tableting [28, 29]
Parameters primarily are considered during multilayer tableting. It is totally depending upon
handling and experience of such multi-dosing tablet press.
Dwell time
It is the contact between punch head and compression roller. If shorter the first layer-dwell
time, which results into pours, aeration, capping and hardness problems. It may be removed
the mistakes by reducing the turret-rotation speed or by extending the dwell time.
Cohesiveness
When the first layer is compressed at a very high compression force, bonding between layers
is severely controlled. Various bilayer formulations necessitate a first layer compression force
NMT 3 Kpor 30 N to maintain the ability of first layer to bond with the second layer. Thus at
elevated manufacturing speed, the jeopardy of separation and capping increases which can be
minimised by adjusting adequate dwell time at all compression stages.
Risk of separation and capping
It is necessary to avoid risk of separation and capping, by forming correct bonding which can
be attained by the first layer formation at low compression force. Therefore this first layer can
still interact with the second layer during final compression of the tablet.
Cross-contamination
Multilayer tablet machines are equipped with suction nozzles or dust extractor to remove fine
powder or granules to eliminate cross-contamination between the two layers and getting a
clear visual separation between layers. It is very important to remove any powder residue
from the die plate and for this purpose dedicated scraper plate are located before and after
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each die fill, to remove residual powder dust to the outside of the die table, where the high
efficiency suction nozzles are located. [30]
Final compression force
This force is applied on the final bilayer tablet is always more than the compression force on
first layer, which results in suitable bonding of both the layers.
Weight variation
Weight variation occurs some time due to non-uniform flow of granules, incomplete die
filling and lower punch jamming due to excessive fines in final blend and thus these
parameters should be controlled carefully during tabletting.
Weight adjustment
First layer pressure is useful for weight adjustment of second layer. Many formulators use
such technique to achieve desired weight instead of using weight adjustment knob that totally
depends on handling experience of such double rotary press.
Hardness and Thickness
This parameters need to be tightly controlled during final compression because it directly affect the release of active. Many times due to high hardness disintegrating matrix may take time more than limits. The manufacturing of multilayered matrix tablets involves the following steps [31-33]
The manufacturing of the matrix core tablets is performed in a separate step and is usually
done by using a regular rotator press. Since the barrier layer is important to monitor drug
release mechanism, its weight, thickness, and compaction need to be tightly controlled during
final compression. The process includes the following steps.
Dosing of the bottom layer
Transfer of the prepared core
Insertion into die
Dosing of the top layer
Final compression
Ejection.
CONCLUSION
The design feature of multilayered tablets provides unique product performance objectives
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which are otherwise not achievable by conventional tablets, but also brings a new set of
challenges for formulation design, manufacturing process, controls and product life
performance requirements. In addition to manufacturing science challenges, they also add
challenges in establishing relevant regulatory controls to meet the product performance
requirements over the life of the drug product. To meet these requirements a higher level of
understanding in the ingredients and manufacturing variables is critical to manage the risks
associated with product acceptability over the life cycle to avoid batch failures and batch
recall.
Thus, the development and production of quality bi-layer tablets require a comprehensive
understanding of the product and process in order to address challenges in manufacturing
such as accuracy in weight control of each individual layer, de-lamination/layer-separation
during manufacturing and storage, insufficient tablet breaking force and cross-contamination
between the layers (especially for incompatible APIs). The objective of the dosage form is to
ensure that the drugs available to the patient are not only safe and effective, but are also
properly manufactured and packaged to meet the established quality target product profile
over it shelf life. A well-developed product will effectively address these issues by including
appropriate control strategies and establishing the functional relationships of the material
attributes and process parameters critical to the multilayer tablet quality.
REFERENCES
1. Chien YW. Novel Drug Delivery System. By Informa Healthcare. Second Edition; pp.
139-140, 2009.
2. Bogan RK. Treatment options for insomnia - pharmacodynamics of zolpidem extended-
release to benefit next-day performance. Postgrad Med, 2008; 120:161-171.
3. Maroni A, Zema L, Carea M, Sangalli ME. Oral pulsatile drug delivery systems. Expert
Opin Drug. Deliv, 2005; 2(5):855-71.
4. Dalvadi H, Patel JK. Chrnopharmaceutics, pulsatile drug delivery system as current trend.
Asian Journal of Pharmaceutical Sciences, 5(5): 207-30.
5. Shah AC,Britten NJLS, OlanoffJN. Badalamenti, Gelmatrix system exhibiting bimodal
controlled-release for oral drug delivery.Journal ofControl Release,1989; 9: 169- 175.
6. Maggi L, Morgenthaler S, Zimmer R, Shepard T, Conte U. Human evaluation of
Quick/Slow drug delivery technology: a new therapeutic approach. Proceedings of the
www.wjpps.com
283
Nikhil et al. World Journal of Pharmacy and Pharmaceutical Sciences
22nd International Symposium on Controlled Release of Bioactive Materials, Seattle,
USA, 1992; 208- 209.
7. Shiyani B, Gattani S, Surana S.Formulation and evaluation of bilayer tablet of
metoclopramide hydrochloride and ibuprofen. AAPS Pharm Sci Tech, 2008; 9: 818-827.
8. Phaechamud T. Variables influencing drug release from layered matrix system
comprising hydroxypropyl methylcellulose. AAPS PharmSciTech, 2008; 9: 668-674.
9. Rathod RT, Misra D. FDC of montelukast with levocetirizine: focus on bilayer
technology. Journal of Indian Med Assoc, 2009; 107: 562-574.
10. Abdu S, Poddar SS.A flexible technology for modified release of drugs: multi layered
tablets. Journal of Control Release, 2004; 97: 393-405.
11. Park JS, Shim JY, Park JS, Choi YW, Jeong SH. A novel three-layered tablet for
extended release with various layer formulations and in vitro release profiles. Drug Dev
Ind Pharm, 2011; 37: 664-672.
12. Shaikh RP, Pillay V, Choonara YE, Ndeseudo VMK, Cooppan S. A review on multi
responsive membranous systems for rate-modulated drug delivery.AAPS
PharSciTech,2010; 11: 441-459.
13. Aboelwafa AA, Basalious EB. Optimization and In vivo pharmacokinetic study of a
novel controlled release venlafaxine hydrochloride Three-Layer Tablet. AAPS Pharm Sci
Tech., 2010; 11: 1026-1037.
14. Krishnaiah YSR, Karthikeyan RS, Gouri Sankar V, Satyanarayana V.Three-layer guar
gum matrix tablet formulations for oral controlled delivery of highly soluble
trimetazidinedihydrochloride. Journal of Control Release, 2002; 81: 45-56.
15. Choi YW, Cui JH, Lee BJ.Formulation, release characteristics and bioavailability of
novel monolithic hydroxyl propylmethylcellulose matrix tablet containing
acetaminophen. Journal of Control Release, 2005; 108: 351-361.
16. Namdeo B. Barrier layers in multilayered tablets. Express Pharma, 2008.
17. Kulkarni A, Bhatia M. Development and evaluation of bilayer floating tablets of atenolol
and lovastatin for biphasic release profile. Iran. J Pharm Res., 2009; 8:15-25.
18. Gautam CS, Saha L. Fixed dose drug combinations (FDCs): rational or irrational: a view
point. Br J ClinPharmacol, 2008; 65: 795-796.
19. WIPO Patent WO2007132281A1
20. US Patent 4874388
21. USPatent 7014867
www.wjpps.com
284
Nikhil et al. World Journal of Pharmacy and Pharmaceutical Sciences
22. Qiu YN. Chidambaram K. Design and evaluation of layered diffusional matrices for zero-
order sustained-release. Journal of Control. Release, 1998; 51: 123-130.
23. Vyas SP, Khar RK. Controlled rug delivery. Concept and advances, 1st edition, Vallabha
prakashan, Delhi, pp: 267-347, 2002.
24. Lachman Leon, Lieberman Herbert A. Compression coated and layer tablets. In:
Pharmaceutical Dosage Forms: Tablets. 2nd Edition, Marcel Dekker, Inc.New York, Vol
1, pp: 247-84, 2002.
25. Streubela A, Siepmannb J, Peppasc NA, Bodmeier R. Bimodal drug release achieved with
multi-layer matrix tablets: transport mechanisms and device design.
26. Nirmal J, Saisivam S, Peddanna C, Muralidharan S and Nagarajan M. Bilayer tablets of
atorvastatin calciumand nicotinic acid: formulation and evaluation Chem Pharm Bull,
2008; 56: 1455-1458.
27. Efentakis M, Peponaki C. Formulation study and evaluation of matrix and three-layer
tablet sustained drug delivery systems based on carbopols with isosorbite mononitrate.
AAPS Pharm SciTech. 2008;9: 917-23.
28. LaForce C,Gentile DA, Skoner DP.A randomized, double-blind, parallel group,
multicentre, placebo-controlled study of the IJPCBS, 2013; 3(3): 887-893.
29. Maggi L,Segale L, Conti S,Ochoa Machiste E. Conte U. Preparation and evaluation of
release characteristics of TabGum, a novel chewing device. Eur J Pharm Sci. 2005; 4:
487-93.
30. Park CR, Munday DL. Development and evaluation of a biphasic buccaladhesive
tablet fornicotine replacement therapy. Int J Pharm., 2002; 237:215-26.
31. Sungthongjeen S, Sriamornsak P, Puttipipatkhachorn S.Design and evaluation of
floating multi-layer coated tablets based on gas formation.Eur J Pharm Biopharm., 2008;
69:255-263.
32. Rama Prasad YV, Krishnaiah YSR, Satyanarayana S. Invitro evaluation of guar gum as a
carrier for colon-specific drug delivery. Journal of Control Release,1998; 51: 281-287.
33. Jain NK, Kulkarni K, Talwar N. Controlled-release table formulation of Isoniazid.
Pharmazie, 1992; 47: 277-278.
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