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Biopharmaceutics
109
In fact, the wet granulation method predominantly involves a plethora of steps each of which would mostly
infl uence such crucial and important functionalities, such as:
∑ dissolution method,
∑ duration of blending,
∑ details of method selected,
∑ total times involved and
∑ precise temperature of drying.
Precompression Method In the precompression method frequently used to manufacture compressed tables,
one makes use of the ‘dry formulated mass’ duly compressed into a ‘slightly oversized tablet or slug’, that
is consequently ground to a uniform size for recompression into the fi nished tablet.
Necessity of Precompression Method
1. It is mostly suitable with such drugs which get decomposed rapidly because of exposure to mois-
ture and heat.
2. Besides, the precompression method specially circumvents the disastrous effect of moisture and
heat and, thus, enormously saves man hours and time factor because it completely avoids/elimi-
nates such operations as wet mixing, drying and granulating.
The following are the advantages:
∑ It enables the compression into a ‘singles tablet’ of acids and bicarbonate, e.g., effervescence tablet.
∑ The formulation must be designed to possess a reasonable degree of adhesiveness, moderately dense
and easy fl ow.
∑ The latest progressive advantage in the precompression method being the inclusion of ‘spray-dried
lactose’, for such ‘active drugs’ with a small dose, as a specifi c diluent.
Applied Compression Force The term compression refers to the process of rendering a discrete quan-
tity of substance more dense or more compact, e.g., solid-granules to ‘tablets’, and gases (LPG, CNG,
Ammonia, N
2
, O
2
etc.) to liquid states.
Table 2.7 Some selected granulation agents used in the wet granulation method
S. No. Granulating agents Granulating fl uid added (%)
1 Sugar 70–85
2 Glucose 25–50
3 Acacia 10–20
4 Gelatin 10–20
5 Cellulose derivatives 5–10
6 Starch 5–10
7 Polyvinylpyrrolidone (PVP) 3–15
8 Alcohol [95% v/v] qs
9 Deionized water qs
Essentials of Biopharmaceutics and Pharmacokinetics
110
It is, however, pertinent to state here that the specifi c and critical compression (or compaction) force
duly used in the meticulous ‘tableting process’ may predominantly infl uence such physical characteristic
features of ‘Tablets’:
∑ density,
∑ porosity,
∑ hardness,
∑ disintegration time and
∑ dissolution test.
Figure 2.35 illustrates explicitly the precise infl uence of the applied compression force upon the ensu-
ing dissolution rate of tablets.
There are four different situations that may arise due to the application of varied compression forces,
namely:
Situation 1: Application of higher compression force enhances the density and hardness of tablet thereby
decreasing the porosity and, therefore, gives rise to a number of changes in the status of a ‘tablet’, such as:
∑ reduces penetrability of solvent inside the body of the tablet,
∑ minimizes wettability by producing a fi rmer and rather more effective sealant layer by lubricant and
∑ provides much tighter bonding created in between the particle crevices,
which concertedly result in exhibiting the dissolution rate of tablets as depicted in Curve ‘A’ (Figure 2.35).
Situation 2: Usage of higher compression forces would cause deformation fracture, or even crushing
of drug particles right into respective:
∑ smaller particulate matters and
∑ help in the conversion of a spherical granule into a ‘disc-shaped particulate matter’ having a large
enhanced surfaces area effectively.
Ultimately, it gives rise to an increased dissolution rate of the ‘tablets’ as depicted in curve ‘B’ (see
Figure 2.35).
Situations 3 and 4: A logical, feasible and possible combination of the two curves ‘A’ and ‘B’ may
also be accomplished as illustrated duly in curves ‘C’ and ‘D’ (Figure 2.35).
Figure 2.35 Varied infl uence of compression forces [A→D] on the dissolution rate of tablets.
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Note
Conclusively, it may be pertinent to mention here that the specifi c and critical infl uence of the
ensuring compression force upon the ‘rate of dissolution’ is invariably a diffi cult task to predict
precisely. Perhaps, it could be achievable by carrying out a thorough investigative study particu-
larly in each individual formulation so as to ensure overwhelmingly better dissolution and bio-
availability of the ‘tablet’.
Biopharmaceutics
111
Packing of Capsule Contents Just similar to the applied compression force (see Applied Compres-
sion Force in this section) particularly applicable to only ‘tablets’, the so-called ‘packing density’ in the
particular instance of the ‘Capsules Dosage Form’ (or ‘Capsules Product Form’) may either inhibit or
promote dissolution.
Critical Observations: These essentially comprise the following:
1. The actual diffusion of GI fl uids into the tightly fi lled capsules prominently gives a high pressure
very much within the capsule itself thereby causing
∑ fast and rapid bursting and
∑ immediate dissolution of its contents.
2. It has been amply proved and demonstrated that such capsules having excessively fi ner particles and
intense packing do exhibit the following typical and characteristic features:
∑ defi nite poor drug release profi le,
∑ reduced dissolution rates on account of the appreciable decrease in the poor size of the com-
pact and
∑ distinctly poor permeability seen in the GI fl uids.
7.5.2. Pharmaceutic Ingredients
The pharmaceutic ingredients are also referred to in the literature as ‘pharmaceutic excipients’ or some-
times also as ‘formulation factors’.
In a broader perspective, an ‘active drug’ is administered very rarely in its original form into a human
system. The most commonly adopted means being the well known conversion product form (or dosage
form) invariably administrated via an appropriate route, viz. oral, IM, IV, SC, CSF and the like.
Obviously, the orally administered solid-dosage form does contain an array of ‘excipients’, also termed
as the ‘non-drug constituents of a formulated drug product’. The very purpose of incorporating the
‘excipients’ are as enumerated below:
∑ accomplish and ascertain acceptability,
∑ ensure enough physicochemical stability in the course of extended shelf life of product,
∑ perfect uniformity of composition and dosage strength and
∑ optimized bioavailability and functionability of the intended drug product.
Characteristics of Excipients: The characteristics essentially consist of the following:
1. In spite of the inert nature of excipients and their superb utilities in the formulated drug product,
they may infl uence the normal absorption of drugs.
2. One may usually encounter more complex problems with regard to absorption pattern and bio-
availability profi le, when more number of excipients are used in the dosage form.
3. The most commonly and extensively used excipients in a variety of ‘drug products’ are as given below:
∑ diluents (fi llers),
∑ binders and granulating agents,
∑ vehicles,
∑ lubricants,
∑ disintegrants,
∑ emulsions,
∑ coatings,
∑ surfactants,
∑ suspending agents,
∑ buffers,
Essentials of Biopharmaceutics and Pharmacokinetics
112
∑ colorants,
∑ complexing agents,
∑ sweeteners,
∑ stabilizers,
∑ preservatives and
∑ crystal-growth inhibitors.
Some of the above-mentioned excipients (or pharmaceutic ingredients) will now be discussed briefl y
in the following sections.
Diluents (or Fillers) Diluents may be defi ned as ‘any substance added to dilute or make less concen-
trated: solids, e.g., sucrose, lactose, starch; liquids, e.g., alcohol, water, glycerine; and semi-liquid, e.g., liquid
glucose, sorbitol’.
In usual practice, the diluents are incorporated invariably in tablet and capsule formulations in such a
situation when the required dosage regimen is not enough to produce the ultimate necessary required
bulk. Moreover, the diluent could be either organic or inorganic in nature.
1. Organic Diluents: Carbohydrates occupy a very vital and important covert status as a diluent in
solid-dosage formulations.
Carbohydrates designate the most important class of organic diluents which essentially comprise
microcrystalline cellulose, starch and lactose.
Examples
Mechanism of Action: These carbohydrates behave as hydrophilic materials and are found to be
extremely useful in distinctly substantiating and promoting specifi c dissolution of not only hydrophobic
drugs, but also sparingly water soluble drugs, e.g., triamterene (a potent diuretic) and spironolactone.
1. Steroidal diuretic/anti-hypertensive: It predominantly forms a suffi cient coat upon the hydro-
phobic surface of the drug particles, rending them hydrophilic in nature genuinely.
2. Inorganic diluents: Dicalcium phosphate (DCP) represents one of the most versatile and abun-
dantly used inorganic diluents.
Mechanism of Action: The most classical drug–diluent interaction that causes seriously poor bioavail-
ability profi le may be observed with DCP and tetracycline (a potent broad-spectrum antibiotic). It is due
to the critical production of a ‘divalent Ca–tetracycline complex’, which is found to be soluble sparingly
and, hence, poses an unobservable feature.
Binders and Granulating Agents Binder refers to a substance being added to a formulation so as to
cause particles to adhere (usually for ‘granulation’ and subsequent tablet compression). It is also some-
times called as ‘adhesives’.
In true sense, the binders and granulating agents are used most frequently to held powders together to
form granules or to afford and promote adequate ‘cohesive compacts’ specially for directly compressible
materials, and also to render the ‘tablet’ perfectly in an ‘intact state’ after compression.
Following are certain typical examples of commonly employed binders and granulating agents:
∑ Naturals, synthetic and semi-synthetic polymeric materials—e.g., acacia, cellulose derivatives,
polyvinylpyrrolidone (PVP) and starch.
Examples
Biopharmaceutics
113
∑ Hydrophilic (aqueous) binders—usually display better dissolution profi le with relatively spar-
ingly wettable drugs by providing critically the desired hydrophilic characteristic features to
the surface of granules.
∑ Strong binders—need to be used in the ‘tablet formulation’ with great care and caution, since
their correct proportion in the formulation itself remains extremely critical and important.
PEG-6000 proved to be a deleterious ‘binder’ for the ‘active drug’ phenobarbatone because it does from
a sparingly soluble complex with the drug itself.
Besides, excessive quantum of these ‘binders’ invariably enhance the hardness, minimize the disinte-
gration rate and reduce the dissolution rates of ‘tablets’.
∑ Non-aqueous binders—also apparently lower the drug dissolution to an appreciable extent, e.g.,
ethyl cellulose.
Vehicles A vehicle is referred to as a carrier or inert medium used as a solvent (or diluent) in which a
medicinally active agent is duly formulated and/or administered.
In other words, vehicle or solvent system designates the major component of the particular class of prod-
ucts know as ‘orals and parenterals’. There are in all three different categories of vehicles that are used
generally, such as:
1. aqueous vehicles, e.g., syrup, water.
2. non-aqueous water miscible vehicles, e.g., sorbitol, glycerol and propylene glycol.
3. non-aqueous water immiscible vehicles, e.g., edible vegetable (double refi ned) oils.
Salient Features: These essentially include the following:
1. A drug’s bioavailability profi le solely rests upon the ability of the vehicle in exhibiting its enormous
degree of miscibility in the biological fl uids.
2. Both ‘aqueous’ and ‘non-aqueous water miscible vehicles’ are duly observed to be miscible with
the body fl uids; and the drugs are adequately absorbed from there in an effi cient manner. Thus, it
promotes overwhelmingly the rapid absorption of the ‘active drugs’.
3. However, it may be observed quite frequently that an ‘active drug’ which happens to be more
soluble in ‘water miscible vehicles’ (viz. propylene glycol–performing as a ‘co-solvent’) defi nitely
displays much improved and better bioavailability profi le.
Note
Importantly, careful and judicious dilutions of such vehicles along with the body fl uids (e.g., blood
CSF, plasma) may result in the critical precipitation of the ‘active drug’ as fi ne particulate matters
that would also undergo rapid dissolution phenomenon.
4. Potential ‘solubilizer’, viz. Tween 80, is at times used invariably to augment and promote the
desired solubility of an ‘active drug’ in the aqueous vehicles.
5. Interestingly, one may take cognizance of the fact that in non-aqueous water immiscible vehicles, the
observed rate of drugs absorption depends almost exclusively upon its due and effective partition-
ing taking place from the respective ‘oil-phase’ to the corresponding ‘aqueous body fl uids’, that may
categorically turn out to be a ‘rate-limiting factor’.
Essentials of Biopharmaceutics and Pharmacokinetics
114
6. The critical and authentic absorption of drug substances is prominently modulated by the ‘viscos-
ity of the vehicles’. It, therefore, gives rise to a diffusion process right into the bulk of GI fl uids,
thereby rendering the critical absorption of an ‘active drug’ from the core of a ‘viscous vehicle’
rather slower in pace.
Lubricants (or Anti-frictional Agents) Lubricants or Anti-frictional Agents are defi ned as ‘slippery, fi ne
powder usually mixed with tablet granules to facilitate a uniform fl ow of drug granules into a tablet die,
and to check and prevent the sticking problems usually encountered during compression’.
In other words, the ‘lubricants’ judiciously and intelligently incorporated in ‘tablet formulations’ do
muster adequate and paramount support to the fl ow of granules, minimize ensuing interparticle friction
and adhesion (sticking) of particles to the dies and punches in a high-speed PC-based tableting machine.
These essentially comprise the following:
1. Metallic stearates, saturated fatty acids (hydrogenated vegetables oils), e.g., magnesium stear-
ate, stearic acid, hydrogenated fatty acids, which are known to cause distinct inhibition of
wettability, sealing of moisture into tablet, and its respective disintegration and dissolution.
Mechanism: The lubricants help in the effective coating of the added disintegrants when
blended simultaneously in the formulation of a tablet. However, the lubricant may be incor-
porated in the fi nal stage, i.e., just prior to the compression of the granules into a ‘tablet’.
2. Soluble lubricants, e.g., carbowaxes and sodium lauryl sulphate [SLS], which are obviously
the best alternative lubricants to prompt the dissolution of an ‘active drug’ effi caciously.
Examples
Disintegrants (or Disintegrating Agents) Disintegrant is defi ned as ‘a substance added to a tablet
granulation during its preparation and after granulation to facilitate the breaking apart of the tablet into
granules, and the splitting apart of the granules on being subjected to the fl uids of the GI tract’.
In true sense, these substances, viz. dried starch, microcrystalline starch, critically overcome the cohe-
sive strength of tablet, and eventually break them up immediately on contact with water, which being a
cardinal pre-requisite to the process of ‘tablet dissolution’.
Important Points These essentially include the following:
1. Most disintegrants are usually hydrophilic in nature.
2. Use of lesser disintegrant in a tablet formulation may signifi cantly lead to an obvious reduced bio-
availability.
3. Adsorbing disintegrants, e.g., veegum, bentonite, must be avoided particularly with low-dosage
active drugs, such as potent alkaloids, steroids, digoxin, as a relatively large quantum of dose gets
adsorbed permanently, whereas, only an extremely negligible fraction remains available for absor ption
ultimately.
Note
Importantly, it may be observed that ‘microcrystalline cellulose’ though serves as a highly
effective disintegrant, nevertheless, at higher compression forces, it would effectively reduce
the drug dissolution.
Biopharmaceutics
115
Coatings It has been duly observed on a broader perspective that the various coatings do exert a
marked and pronounced harmful effect upon the ensuring drug dissolution from a tablet-dosage form
follows the under-mentioned order:
Enteric coat > sugar coat > non-enteric film coat
It is, however, pertinent to state here that the critical as well as specifi c ‘dissolution characteristic
feature’ of certain typical coating materials do alter on ageing, such as the so-called ‘shellac-coated
tablets’ upon storage for a long period, invariably get dissolved more gradually in the intestinal passage.
The above discrepancy may be easily circumvented by adding a small quantum of PVP in the ‘coating
formulation’ judiciously.
Surfactants It is indeed a common practice to make use of the surfactants widely and abundantly in
a variety of ‘tablet formulations’ specifi cally as wetting agents, emulsifi ers and solubilizers.
Mechanism: The exact and precise mechanism of action of the surfactant upon their infl uence on
drug absorption seems to be rather complex in nature. Importantly, the surfactants could either increase
or decrease critically the rate of drug absorption accomplished by the following methods:
∑ interaction with the ‘active drug’,
∑ interaction with the ‘membrane’ and/or
∑ interaction with both ‘drug and membrane’.
Nevertheless, the specifi c mechanisms that predominantly enhance only the absorption of the ‘drug’ by
using surfactants are as follows:
∑ Specifi c augmentation in both wetting and dissolution of the ‘active drugs’,
∑ Improved contact of drug with membrane for better absorption signifi cantly and
∑ Defi nite increased membrane permeability of the ‘active drug’.
Note
The advantageous benefi ciary overall effect of the surfactants have been thoroughly investigated
and evaluated specifi cally at the ‘pre-critical micelle
67
concentration levels’.
Physiological Surfactants: Interestingly, the physiological surfactants
68
do help in the much needed efforts
to promote and substantiate the remarkable absorption of the hydrophilic drugs, namely:
∑ steroidal drugs (viz. testosterone, androsterone etc.),
∑ fat-soluble vitamins (viz. vitamins A, D, K) and
∑ griseofulvin,
by virtue of their highly specifi c micellar solubilizing characteristic feature.
Surfactants Causing Reduced Absorption of Active Drug(s): It has been proved beyond any
reasonable doubt that one many defi nitely note a marked decrease in the absorption of active drug(s) by
the critical presence of certain ‘surfactants’, which may be duly caused on the account of
67
Micelle: It refers to the agglomeration amphiphillic surfactant molecules in a dispersion medium (solvent) hav-
ing a diameter of the order of 50 Å or equivalent to 5 nm.
68
Physiological Surfactants: Bile salts (anionic in nature), e.g., alkali salts of bile: sodium glycocholate, and
sodium taurochlolate; Lysolecithin (non-ionic in nature): obtained duly from lecithin via the action of an enzyme
present in cobra venom.
Essentials of Biopharmaceutics and Pharmacokinetics
116
∑ adequate creation of absolutely unabsorbable ‘drug-micelle complex’ at the critical surfactant con-
centration prevailing very much at a level just above the ensuing ‘critical-micelle concentration’, and
∑ excessive ‘surfactant concentration’ is prone to cause defi nite laxative/purgative action.
Buffers A buffer refers to a system containing chemical constituents that eventually resist small changes
in H
+
ion and OH
−
ions concentrations, designed to keep the pH relatively constant.
It may be observed that the buffers do sometimes prove to be really benefi cial creating the prefect
congenial environment for accomplishing drug dissolution, e.g., buffered-aspirin tablets.
Nevertheless, contrarily there are certain specifi c buffer systems comprising K
+
ions that critically
inhibit the absorption of the drug, e.g., sulphanilamide, vitamin B
2
.
Mechanism: The most probable mechanism proposed for such an activity is the crucial up take of
fl uids of the intestinal epithelial cells by the virtue of which the following two actions come into play
in a sequential manner:
∑ reduction of the effective-drug concentration in the tissue,
∑ substantial decrease in the rate of absorption the active drug.
Interestingly, the observed inhibitory effect of certain viable and commonly used buffer cations upon
the rate of drug of transfer is as given in the following order:
4
K NH Li > Na TRIS
++ + +
>> >
Note
It is, therefore, an absolute necessity to have in a given ‘buffer system’ for a salt of an ‘active drug’
must contain the same cation as that of the drug salt, so as not to introduce any additional cations.
Colorants It may be amply observed that even the actual presence of a very low concentration of a
water-soluble dye could afford an ‘inhibitory effect’ upon the critical rate of dissolution of quite a few
crystalline active drugs.
Mechanism: This actually takes place due to the dye molecules being strategically adsorbed upon
the ‘crystal faces’, thereby giving rise to the ultimate inhibition of the drug dissolution appreciably.
Brilliant-blue (a dye) signifi cantly reduces the dissolution of sulphathiazole.
Example
Note
1. There are certain dyes that specifi cally cause inhibition of the ‘micellar solubilization effect’
of the ensuing bile acids (viz. cholic, glycocholic and taurocholic acids) that may categori-
cally impair the critical absorption of certain hydrophobic drugs, e.g., steroids.
2. Importantly, some cationic dyes are found to be more reactive vis-à-vis the anionic dyes per-
haps on account of their more prominent ability for absorption upon the primary particles.
Biopharmaceutics
117
Complexing Agents It has been duly established that the extent of complex formation has exploited
meticulously and advantageously to bring in certain effective changes in a ‘drug’:
∑ physicochemical characteristic features and
∑ biopharmaceutical properties.
Thus, a duly ‘complexed drug entity’ should possess the following typical characteristic features, namely:
∑ changed stability,
∑ altered solubility,
∑ different molecular size,
∑ altered partition coeffi cient and
∑ changed diffusion coeffi cient.
Fundamentally, these aforesaid complexed-drug molecules are proved to be absolutely inert pharma-
cologically and, therefore, should have the ability to undergo critical dissociation either at the absorption
site or eventually soon after absorption right into the systemic circulation system.
Following are some typical and vital examples wherein the ‘complexation’ phenomenon has been
exploited obviously to increase the extent of drug bioavailability to an appreciable level, such as:
1. Increased GI absorption of ‘Heparin’ (an anti-coagulant) due to increased membrane per-
meability in the critical presence of EDTA that specifi cally chelates the bivalent cations, viz.
Ca
2+
and Mg
2+
, from the membranes.
2. Caffeine—PABA Complex: It has essentially an enhanced lipophilicaly for much
improved and better membrane permeability.
3. Ergotamine Tartrare—Caffeine Complex and Hydroquinone Dioxin Complex:
These two complexes do show an increased degree of dissolution due to the formation
of soluble complexes.
Examples
Crystal Growth Inhibitors Besides maintaining and sustaining the primary physical characteristic
features of an ‘active drug’ duly present in a suspension, the very role of the so-called crystal-growth
inhibitors, e.g., PVP and PEG, do go a long way in affording the inhibition for the conversion of a ‘high-
energy metastable polymorph’ right into
∑ less soluble,
∑ fairly stable polymorph.
7.6. Drug Absorption vis-à-vis Patient-Related Factors
It is, indeed, pertinent to mention in the very outset that before undertaking the discussion related to
the specifi c patient-related factors which critically infl uence the bioavailability profi le of a drug, one
may have to understand vividly the anatomy and physiology of the GI tract as illustrated in Figure 2.36.
Mouth (pH 6.8): Lipophilic basic and neutral drugs get absorbed directly into the systemic circulation.
Stomach (pH 1.3): Lipophilic acidic and neutral drugs get absorbed but much lesser than that from
intestine.
Essentials of Biopharmaceutics and Pharmacokinetics
118
Small intestine (pH 5–7.5): Major site for the absorption of all types of drugs (viz. acidic/basic/
neutral/lipophilic).
Large intestine (pH 7.5–8): All kinds of drugs get absorbed but only to a lesser degree.
Rectum (pH 7.5–8): All types of drugs get duly absorbed; almost half of the absorbed drug enters
directly into the systemic circulation.
Liver: Designates a major site for drug metabolism, e.g., fi rst-pass metabolism.
Bile (pH 7.8–8.6): Helps in the critical absorption of lipophilic drugs; the actual route for ‘active
secretion’ of polar drugs/metabolites.
GI tract: It essentially consists of a plethora of vital components that modulate prominently such
primary functions as
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Figure 2.36 A schematic representation of the GI tract and various important sites of drug absorption.