Biopharmaceutics

Biopharmaceutics

Interdependence of biological aspects of the human and physico-chemical principles that governs the preparation and behavior of formulation.

Important parameters

Physical-Chemical parameter:SolubilityLipophilicity/HydrophilicitySalt for and polymorpsChemical stabilityparticle and powder propertiespKa and ionization

Important parameters

Formulation principlesBiological principles

AbsorptionDistributionMetabolismElimination

Molecular and physicochemical properties impacting oral absorption

Lipinski’s Rule of 5: Poor absorption/ permeation may occur if MW>500lop P>5H- bond donors (-OH, -NH)>5 H-bond acceptors

(-H, -N)>10

Veber suggested that polar surfaces and molecular flexibility are good predictor for better bioavailability (independent of MW).

Increased molecular rigidity decreases polarity.

Chirality

Stereoisomers have similar physicochemical parameters and passive processes.

Absorption may be stereospecific when mediated by carrier molecule.

Chiral API+ chiral excipient may give stereoselective

release mechanism.

Dissolution

The dissolution rate for poor soluble drug according to Noyes-Whitney equation is:

dc/dt=[DA/hv]*(Cs-C) D=diffusion constant H=thickness of diffusion layer A=surface area v=volume of dissolution media Cs=saturated solution concentration C=concentration of drug in solution

Solubility

Factors governing poor aqueous solubility are: heat of solvation and heat of fusion

Solvation energy- Energy associate with dissolving solute in water :evaluated by log P

Heat of fusion- additional energy required to liberate molecule from its crystal lattice before it can dissolve (for crystalline compound): evaluated by MP.

If high MP and high heat of fusion; then poor solubility. (if heat of fusion is surpassed by heat of solvation, then solubility may change)

pH-solubility profile

Dissociation constant-pKa; intrinsic solubility and salt solubility are determining factors defining solubility profile.

Effect of temperature on solubility

The relationship between solubility and temperature is defined by van’t Hoff’s equation.

ln s= ΔH/R(1/T)+ constant Where s=molar solubility, T=temperature, R=ideal gas constant; ΔH=heat of solvation.

For organic compounds, it has been found that solubility difference between 25 and 37°C are typically about two times.

It is most important to study temperature effect for formulation development, particularly for liquid dosage form.

Solubility in gastric and intestinal fluid

Solubility of drug is affected by presence of food and bile salts, lecithin and monooleins.

Supersaturation in intestinal fluid is important for compound with poor intrinsic solubility. HPMC enhances absorption of several poor soluble drug.

FaSSIF and FeSSIF are used to study realistic impact of solubility on absorption.

Solubility as limiting factor for absorption

According to Lipinski, solubity of LT 10µg/ml will have effect on absorption.

MAD=S*Ka*SIWV*SITT; where MAD=maximum absorbable dose (mg); S=solubility (mg/ml); Ka=absorption rate constant (1/min); SIWC=small intestinal water volume (ml, mostly 250); SITT-small intestine transit time (min, mostly 270 min)

MAD is concept tool that relates solubility requirement for oral absorption to dose, permeability, GI volume and transit time.

Determination of solubility

Traditional shake flask method.A Powder-XRD run with wet and dry

sample gives hydrate information of residual solid.

For poorly soluble compound, excess amount of material is added and then vortexing with teflon ball/sonication is done. Amorphous compound can be added to create supersaturation.

Care should be taken to prevent conversion to free form from salt form. If solid salt is not available than in situ salt screening method is used to check solubility.

Kinetic solubility is determined by adding compound’s DMSO solution to aq. buffers. It helps to determine poor soluble compounds early. Usefulness and limitation shall be considered.

Prediction of solubility

Yalkowsky and Valvani have introduced a model for nonelectrolytes:

Log S=0.5-0.01(MP-25)-log P;

where MP is melting point effect for solid phase effect and log P liquid phase effects.

Chemical stability

The compound needs to be stable for better shelf-life and bioavailability.

For ionizable drug, pH-stabiity profile is important for knowing impact of absorption.

The acidic degradation shall also be checked for improving bioavailability.

Degradation by hydrolysis, oxidation and reduction.

Stability in compound intestine is affected by pH, enzymes, bacterial flora.

Solid state properties

PolymorphismAmorphous materialParticle size

Polymorphism

Compound may exist in different polymorphic form, solvates or hydrates.

Polymorph and pseudo-polymorph have different solubility, crystal shape, dissolution rate and thus possibly different absorption rate.

Thermodynamically most stable form shall be selected for dosage form development.

Crystallization- re-crystallization are used to determine polymorph, solvates or hydrate form.

Amorphous material

Amorphous form has highest free energy having impact on solubility and bioavailability.

The pharmaceutical process have impact on product quality.

Amorphous form can have 1000 fold solubility, but it is difficult to measure their true solubility as they are not stable in any solvent.

If the drug in amorphous form is miscible with polymers in solid dispersion, they may be physically stable even at accelerated condition.

Particle size

Particle size reduction increases surface area available for dissolution, and thus absorption.

At submicron level, particles may agglomerate to reduce the free energy of system which can be prevented by addition of surfactant.

Ionization and pKa

For ionizable compound, the aq. Solubility of ionized species is higher than the unionized species because of higher polarity.

Ionizable functional group and magnitude of pKa determines weather the compound will be in ionized form at physiological form.

For transportation of drug, it should be in solution and non-ionized.

But non-ionized form have higher lipophilicity, and thus lower solubility.

At physiological pH, the drug will be in both ionized and unionized form. The non-ionized spp. will get absorbed and thus it is regenerated for maintenance of equilibrium between ionized-non ionized spp.

Biological factors

Physiological Principles-Absorption

Drug must be absorbed across a biological membrane to give a pharmacologic response.

Transport across the membrane (Passive/Active) is impacted by ionization equilibria, partitioning into and diffusion across lipophillic membrane along with interaction with transporter system.

Lipophilicity affect membrane permeability while solubility affects GI absoprtion.

Passive diffusion

Passive transport is movement of molecule from higher concentration to lower concentration.

It is governed by Fick’s firest law:J=dM/A dt= D(C1-C2)/h, where J=flux,

M=drug mass, A=area, t=time, D=diffusion coefficient, C1=drug conc. in intestinal lumen, C2=conc. in blood side

Carrier mediated transport

Depending on the direction and category of solutes, they can also be classified in to uniporter, symporter and antiporter.

Based on energy involved, they can be classified in to facilitated diffusion and active transport.

Facilitated diffusion

Carrier protein are involved no need of energy.

Same as passive diffusion, transport direction of facilitated diffusion depends on solute concentration gradient.

Active transport

It is energy consuming process. This form of transport involves specific carrier

proteins present in the cell membranes. Enzymes, temperature, stereo specificity of

molecule can affect it. As this process involves a carrier protein, the

mechanism is saturable at high concentrations and uptake via this route can be inhibited by competing substrates.

Paracellular transport

Drugs can also cross epithelia through gaps (known as gap junctions) between the cells.

This route is governed by passive diffusion and small hydrophilic molecules can pass through these gap junctions.

Transport across the epithelia can be enhanced using penetration enhancers which can damage the gap junctions; however possible toxicity implications should be considered with such methods.

Endocytosis

This process involves internalisation of substances by engulfment by the cell membrane which forms membrane-based vesicles within the cell, known as endosomes. This allows larger molecules or particulates to enter the cell.

There are several types of endocytosis: Receptor-mediated endocytosis; Adsorptive endocytosis; Pinocytosis; Phagocytosis

Pore transport

Very small molecules may also be taken up through aqueous pores that are present in some cell membranes. These are ~0.4 nm in diameter so this transport mechanism is very restrictive. Only very small hydrophilic drugs can enter cells via this route.

Efflux

Substances can also be pushed back out of cells by an energy-dependent efflux system. There are various apical transmembrane proteins which can transport drugs out of the cell.

Drugs that are subjected to efflux processes include cytotoxic drugs such as taxol, steroids, immunosuppressants and antibiotics.

The genetic information for efflux pumps can be contained within chromosomes and/or plasmids. This allows for the efflux pump genes to be passed to various bacterial species. Expression of several efflux pumps in bacteria can lead to multidrug resistance.

Transporters and Their Substrate

Transporter Substrates Tissue

Lipid transporters/bile acid transporters

Fatty acids, cholesterol, (lipophilic drug?)

Monocarboxylate transporters (organic anionic transporters)

Salicyclic acid, pravastatin, NSAIDs, some fluroquinolones

Organic cation transporters Epinephrine, choline, dopamine, guanidine, antiarrhythmics, several antihistaminics

Nucleoside transporter Nucleoside analogues used as antiviral and anticancer compounds

Small intestine, kidney

Intestinal dipeptide transporter β-lactam antibiotics, angiotensin-converting enzyme inhibitors, thrombin inhibitors, thyrotropic-releasing hormone

Transporter Substrates Tissue

Amino acid transporters Gabapentin, baclofen, L-alpha-methyl dopa, L-dopa

Intestine, kidney, lung

Vitamin transporters Valproic acid, salicylic acid, penicillins, methotrexate

Kidney, intestine

Phosphate transporter Fosfomycin, phosphonoacetic acid, phosphopropionic acid

P-glycoprotein (P-gp) Etopside, vincristine, paclitaxel, ofloxacin, ciprofloxacin, cyclosporin, peptides, some organic cations, vinblastine

Primary factors influencing absorption

Membrane permeabilitySolubilityDissolution of solid dosage form

Membrane permeability

Effective permeability (Peff): it is dependent on drug concentration for drugs that absorbs through both passive and active transport.

P=J/C=DK/h

Fraction of drug absorbed (Fa): permeability through intestinal wall determines fraction of drug absorbed.

Fa=1-e-2Peff*Tres/Tabs

Absorption rate constant (Ka): permeability and radius of intestine determines absorption

Ka=2P/R

Permeability absorption model

Physicochemical methods: Lipophilicity (Log P/Log D), Absorption Potential, Immobilized artificial membranne

In vitro methods: Animal tissue based methods: Everted Gut technique, Ussing

Chamber, Isolated membrane vesicles Cell based methods: caco-2 cells and others, MDCK cells

In situ methods In vivo models In silico methods

Dissolution of solid dosage form

Drug in solid dosage form must be dissolved before it can be absorbed.

Factors affecting dissolution affect absorption process for mainly poor soluble, high dose drug.

Formulation strategies such as micronization, ionization, surfactants and disintegrants can be used to enhance to increase absorption.

Secondary factors influencing drug absorption

Biological factors of gastro intestinal track Gastric emptying time Surface area GI transit time Intestinal motility Food Blood flow Components, volume &

properties of GI fluid age

Dosage factors influencing absorption Excipients Coating i.e. enteric

coating Surfactants. Etc.

Evaluation of oral drug absorption

Drug absorption assessment using in vivo data Estimation of fraction of drug absorbed using

experimental intestinal permeability in vivoPeff, human=Q(1-Cout/Cin)/2πRL), where Q=perfusion flow rate, Cin=inlet drug conc. Of perfusion tube, R=radius, L=length

Estimation of MAD using in vivo absorption rate constant and drug solubilityMAD=SKaVT, where S=solubility, Ka=absorption rate constant, V=volume, T=time

Estimation of MAD from drug in vivo permeability in humans and drug solubilityMAD=Peff,humanSAT, where Peff,human=drug permeability in human intestine, S=solubility, A=absorption rate, T=transite time in small intestine

Drug absorption assessment using in vitro data In vitro testing conditions for determining drug permeability in

caco-2 cells and in vitro/in vivo permeability correlation

LogPeff, human=constant*LogPeff, caco-2-constant Estimation of fraction of drug absorbed in humans using in

vitro drug permeability in caco-2 cells Estimation of MAD in human based on in vitro data

MAD=Peff, humanSAeffT, where Aeff=effective surface area without considering villi and microvilli

Correlation of oral bioavailability and intestinal permeability between rat and human