1. rational design of drug products
the physical and chemical properties of the drug substance;
the route of drug administration, including the anatomic and physiologic nature of the application site (eg, oral, topical, injectable, implant, transdermal patch, etc);
desired pharmacodynamic effect (eg, immediate or prolonged activity);
toxicologic properties of the drug;
safety of excipients; and
effect of excipients and dosage form on drug delivery.

2. Pharmaceutic Factors Affecting Drug Bioavailability
Considerations in the design of a drug product that will deliver active drug with the desired bioavailability characteristics include
the type of drug product (eg, solution, suspension, suppository),
the nature of the excipients in the drug product,
the physicochemical properties of the drug molecule, and
the route of drug administration.

3. Necessary for optimum stability and solubility of the final product.
Physicochemical Properties for Consideration in Drug Product Design
pKa and pH profile
Necessary for optimum stability and solubility of the final product.
Particle size
May affect the solubility of the drug and therefore the dissolution rate of the product.
Polymorphism
The ability of a drug to exist in various crystal forms may change the solubility of the drug. Also, the stability of each form is important, because polymorphs may convert from one form to another.
Hygroscopicity
Moisture absorption may affect the physical structure as well as stability of the product.
Partition coefficient
May give some indication of the relative affinity of the drug for oil and water. A drug that has high affinity for oil may have poor release and dissolution from the drug product.
Excipient interaction
The compatibility of the excipients with the drug and sometimes trace elements in excipients may affect the stability of the product. It is important to have specifications of all raw materials.
pH stability profile
The stability of solutions is often affected by the pH of the vehicle; furthermore, because the pH in the stomach and gut is different, knowledge of the stability profile would help to avoid or prevent degradation of the product during storage or after administration.

4. Physicochemical Properties for Consideration in Drug Product Design
Solubility, pH, and Drug Absorption
The is a plot of the solubility of the drug at various solubility – pH profile physiologic pH values.
the natural pH environment of the gastrointestinal tract varies from acidic in the stomach to slightly alkaline in the small intestine.
A basic drug is more soluble in an acidic medium, forming a soluble salt. Conversely, an acid drug is more soluble in the intestine, forming a soluble salt at the more alkaline pH.
Solubility may be improved with the addition of an acidic or basic excipient.
Solubilization of aspirin, for example, may be increased by the addition of an alkaline buffer.

5. Physicochemical Properties for Consideration in Drug Product Design
Stability, pH, and Drug Absorption.
The stability–pH profile is a plot of the reaction rate constant for drug degradation versus pH.
If drug decomposition occurs by acid or base catalysis, some prediction of degradation of the drug in the gastrointestinal tract may be made.
For example, erythromycin has a pH-dependent stability profile. In acidic medium, as in the stomach, erythromycin decomposition occurs rapidly, whereas in neutral or alkaline pH, the drug is relatively stable.

6. Physicochemical Properties for Consideration in Drug Product Design
Particle Size and Drug Absorption
The effective surface area of a drug is increased enormously by a reduction in the particle size.
the greater the surface area, the more rapid is the rate of drug dissolution.
Particle size and particle size distribution studies are important for drugs that have low water solubility.
Griseofulvin, nitrofurantoin, and many steroids are drugs with low aqueous solubility; reduction of the particle size by milling to a micronized form has improved the oral absorption of these drugs.

7. Physicochemical Properties for Consideration in Drug Product Design
Polymorphism, Solvates, and Drug Absorption
Polymorphism refers to the arrangement of a drug substance in various crystal forms or polymorphs.
Amorphous forms are noncrystalline forms,
solvates are forms that contain a solvent (solvate) or water (hydrate), and
desolvated solvates are forms that are made by removing the solvent from the solvate.
Polymorphs have the same chemical structure but different physical properties, such as solubility, density, hardness, and compression characteristics.
In general, the crystal form that has the lowest free energy is the most stable polymorph.
Chloramphenicol, for example, has several crystal forms, and when given orally as a suspension, the drug concentration in the body was found to be dependent on the percent of -polymorph in the suspension. The form is more soluble and better absorbed,

8. Comparison of mean blood serum levels obtained with chloramphenicol palmitate suspensions containing varying ratios of and polymorphs, following single oral dose equivalent to 1.5 g chloramphenicol. Percentage polymorph in the suspension.

9. Effect of Excipients on the Pharmacokinetic Parameters of Oral Drug Products
Example
k a
T MAX
AUC
Disintegrants
Avicel, Explotab /—
Lubricants
Talc, hydrogenated vegetable oil
Coating agent
Hydroxypropylmethyl cellulose —
Enteric coat
Cellulose acetate phthalate
Sustained-release agents
Methylcellulose, ethylcellulose
Sustained-release agents (waxy agents)
Castorwax, Carbowax
Sustained-release agents (gum/viscous)
Veegum, Keltrol
This may be concentration and drug dependent.      = Increase,      = decrease, — = no effect, k a = absorption rate constant, t max = time for peak drug concentration in plasma, AUC = area under the plasma drug concentration–time curve.

10. For example, shows that an excessive quantity of magnesium stearate (a hydrophobic lubricant) in the formulation may retard drug dissolution and slow the rate of drug absorption. The total amount of drug absorbed may also be reduced (). To prevent this problem, the lubricant level should be decreased or a different lubricant selected.

11. Dissolution and Drug Release Testing
Dissolution and drug release tests are in-vitro tests that measure the rate and extent of dissolution or release of the drug substance from a drug product, usually in an aqueous medium under specified conditions. In-vitro drug dissolution studies are most often used for monitoring drug product stability and manufacturing process control.
dissolution and drug release testing may be used for:
Batch-to-batch drug release uniformity
Stability
Predicting in-vivo performance
To accessing therapeutic efficacy & bioequivalence.
Requirement for regulatory approval for product marketing
Scale-up and postapproval changes (SUPAC)

12. Dissolution Conditions
dissolution vessels
dissolution vessels range in size from several milliliters to several liters. The shape may be round-bottomed or flat, so the tablet might lie in a different position in different experiments. The usual volume of the medium is 500–1000 mL. (up to 2000 mL)
dissolution medium
0.1 N HCl, phosphate buffer, simulated gastric juice, water, and simulated intestinal juice, depending on the nature of the drug product and the location in the gastrointestinal tract where the drug is expected to dissolve.
temperature
Most dissolution tests are performed at 37°C. However, for transdermal drug products, the recommended temperature is 32°C.
agitation
The most common rotating speed for the basket method is 100 rpm
And for Apparatus 2 are 50 rpm for solid oral dosage forms and 25 rpm for suspensions.

13. Problems of Variable Control in Dissolution Testing
Depending on the particular dosage form involved, the variables may or may not exert a pronounced effect on the rate of dissolution of the drug or drug product. Variations of 25% or more may occur with the same type of equipment and procedure.
The centering and alignment of the paddle is critical in the paddle method.
Turbulence can create increased agitation, resulting in a higher dissolution rate.
Wobbling and tilting due to worn equipment should be avoided.
the basket method is more sensitive to clogging due to gummy materials.
dissolved gas in the medium may form air bubbles on the surface of the dosage form unit and can affect dissolution in both the basket and paddle methods.
In the absence of in-vivo data, it is generally impossible to make valid conclusions about bioavailability from the dissolution data alone. The use of various testing methods makes it even more difficult to interpret dissolution results, because there is no simple correlation among dissolution results obtained with various methods
For many drug products, the dissolution rates are higher with the paddle method. Dissolution results at 50 rpm with the paddle method may be equivalent to dissolution at 100 rpm with the basket method.
the composition of the formulation as well as the process variables in manufacturing may both be important.
No simple correlation can be made for dissolution results obtained with different methods.
the selection of the dissolution method is based on the type of drug product to be tested.

14. Dissolution Apparatus
Name
Drug Product
Apparatus 1
Rotating basket
Tablets
Apparatus 2
Paddle
Tablets, capsules, modified drug products, suspensions
Apparatus 3
Reciprocating cylinder
Extended-release drug products
Apparatus 4
Flow cell
Drug products containing low-water-soluble drugs
Apparatus 5
Paddle over disk
Transdermal drug products
Apparatus 6
Cylinder
Apparatus 7
Reciprocating disk
Rotating bottle
(Non-USP-NF)
Extended-release drug products (beads)
Diffusion cell (Franz)
Ointments, creams, transdermal drug products

15. Conditions that May Affect Drug Dissolution and Release
Drug substance
Particle size
Polymorph
Surface area
Chemical stability in dissolution media
2. Formulation of drug product
Excipients (lubricants, suspending agents, etc)
3. Medium
  pH
 Molarity
Volume
Co-solvents, added enzymes/surfactants
Hydrodynamics
Agitation rate
Shape of dissolution vessel
Placement of tablet in vessel
 Sinkers (for floating products and products that stick to side of vessel)
5. Temperature of medium
6. Apparatus

16. Rate-Limiting Steps in Drug Absorption
Systemic drug absorption from a drug product consists of a succession of rate processes . For solid oral, immediate-release drug products (eg, tablets, capsules), the rate processes include
disintegration of the drug product and subsequent release of the drug,
dissolution of the drug in an aqueous environment, and
absorption across cell membranes into the systemic circulation.

17. In-Vitro–In-Vivo Correlation
An In-vitro in-vivo correlation (IVIVC) has been defined by the Food and Drug Administration (FDA) as "a predictive mathematical model describing the relationship between an in-vitro property of a dosage form and an in-vivo response".
The United States Pharmacopoeia (USP) also defines IVIVC as "the establishment of a relationship between a biological property, or a parameter derived from a biological property produced from a dosage form, and a physicochemical property of the same dosage form".
Typically, the parameter derived from the biological property is AUC or Cmax, while the physicochemical property is the in vitro dissolution profile.
The main role's of IVIVC are:
The first and main role of establishing IVIVC is to use dissolution test as a surrogate for human studies.
Supports and / or validates the use of dissolution methods and specifications.
Assists in quality control during manufacturing and selecting appropriate formulations

18. Various parameters used in IVIVC depending on the level.
In vitro
In vivo A
Dissolution curve
Input (absorption) Curves B
Statistical moments: mean
dissolution time (MDT)
residence time (MRT), mean absorption time (MAT), etc C
Disintegration time, Time to have 10, 50, 90% dissolved, Dissolution rate, Dissolution efficiency
Cmax, Tmax, Ka, Time to have 10,50,90% absorbed, AUC (total or cumulative),

19. Biopharmaceutics drug clarification system.
Class
Solubility
Permeability
Absorption rate control
IVIVC expectation
BCS Class I
High
Gastric emptying
IVIVC expected if dissolution rate is slow than gastric emptying rate otherwise limited or no correlation
BCS Class II
LOW
Dissolution
IVIVC expected if in vitro dissolution rate is similar to in vivo dissolution, unless very high dose
BCS Class III
Low
Absorption (permeability) is rate determining, limited or no IVIVC with dissolution
BCS Class IV
Not defined
(case by case)
Limited or no IVIVC is expected

20. In-Vitro–In-Vivo Correlation
In Vivo Method
Pharmacokinetic Methods:
Indirect measurement of therapeutic effectiveness of the drug
Blood level studies (plasma or serum)
Urinary excretion studies
Assay of other biological material (saliva, CSF, bile, fecal recovery study, etc)
Pharmacodynamic Methods:
Direct measurement of therapeutic effectiveness of the drug
Acute pharmacological response
Therapeutic response
IN-VITRO METHOD
Release characteristics Drug dissolution testing modules
Permeation studies Diffusion model, Everted sac, everted ring technique, caco-2 cells
IN-SITU METHOD
Doluisio method, single perfusion method

22. Failure to correlate in-vitro dissolution to in-vivo absorption
Nature of exceptions:
Drug failed in dissolution test and yet well absorbed ;
No or poor correlation between dissolution (in- vitro bioavailability) and in-vivo absorption
Probable causes:
Complexity of drug absorption process
a product that involves fatty components may retain longer in GI tract;
effect of digestive enzymes play an important role in in-vivo dissolution which may not be adequately simulated in a dissolution media
Weakness of in-vitro dissolution design
e.g. among two sustained-release quinidine gluconate tablets one is completely bioavailable and the other is partially absorbed.
Two models are used:
Model 1: Using acid media as well as acid followed by pH 7.4 interestingly don’t distinguish two products well.
Model 2: Using water or pH 5.4 buffer as buffer media clearly distinguishes two products.
The condition of Model 1 is consistent with physiologic condition of stomach, but the procedure would be misleading as a quality control tool in this case.

24. Biopharmaceutic considerations in Drug Product Designing
Prime Consideration
Safety [finished drug product shouldn’t produce any additional side effect/discomfort due to drug/excipients, excipients should be inactive]
Efficacy [ Drug product should should deliver drug of maximum bioavailability and minimum adverse effect]
Pharmacodynamic consideration
Therapeutic objective
- e.g. drug to treat acute illness should release the drug rapidly like nitroglycerine for angina pectoris;
- for prophylactic treatment of chronic disease like asthma extended release type is preferred
Toxic effect
Adverse reactions

25. Biopharmaceutic considerations in Drug Product Designing
3. Drug consideration
- Physicochemical properties of drug
pH,
aqueous solubility,
polymorphism,
particle size,
dissolution etc.
If a drug has low aqueous solubility and an IV injection is desired then a soluble salt of the drug is preferred

26. Biopharmaceutic considerations in Drug Product Designing
4. Drug product consideration
Pharmacokinetics of drug e.g. therapeutic window;
drug conc. Higher than t.w. may initiate more intense pharmacodynamic & toxic effect whereas below that conc. Could be sub-therapeutic;
Drugs with narrow T.W. Size of dose & dosing frequency to be adjusted precisely to get desired effect with a safe dose
Bioavailability of the drug
Stability of drug in GI tract and intestine, e.g. penicillin G is unstable in acid media of stomach, so addition of buffer or use of enteric coating in the formulation will protect from degradation at low pH;
Some drugs have poor bioavailability due to intense first-pass effect , in this case a higher oral dose (propranolol) or alternative route (Insulin) will be required

27. Biopharmaceutic considerations in Drug Product Designing
Dose consideration
Size of dose
based on inherent property of drug molecule plus its apparent VD which determines target Cp for desired therapeutic effect;
for some drugs several doses are required to allow individualized dosing
Dosing frequency
If a drug has a short duration of action due to short elimination t1/2 then the drug must be given more frequently