2.1 Absorption

Routes of Absorption

The route by which a drug enters the body directly determines its bioavailability and how quickly it reaches therapeutic concentrations. Choosing the right route depends on the drug's chemical properties, the desired speed of onset, and practical considerations like patient compliance.

Oral Absorption

Oral administration is the most common and convenient route. The drug is swallowed and absorbed through the gastrointestinal (GI) tract, primarily in the small intestine where the large surface area of villi and microvilli favors uptake.

• Suitable for drugs that are chemically stable in acidic gastric conditions and can survive first-pass metabolism (hepatic metabolism before reaching systemic circulation)
• Dosage forms include tablets, capsules, syrups, and suspensions
• Onset is relatively slow compared to parenteral routes, typically 30–90 minutes
• The major limitation is that some drugs are extensively metabolized in the gut wall or liver, drastically reducing the amount that reaches the bloodstream

Parenteral Absorption

Parenteral routes bypass the GI tract entirely, delivering drugs directly into the body via injection or infusion.

• Intravenous (IV): Drug goes straight into the bloodstream. Bioavailability is 100% by definition. Onset is immediate.
• Intramuscular (IM): Drug is injected into muscle tissue and absorbed into surrounding capillaries. Onset is typically 10–20 minutes.
• Subcutaneous (SC): Drug is injected beneath the skin. Absorption is slower than IM due to lower blood flow.

Parenteral administration is critical when rapid onset is needed (e.g., emergency situations) or when a drug is destroyed in the GI tract (e.g., insulin).

Topical Absorption

Topical administration applies drugs directly to the skin or mucous membranes for local or systemic effects.

• Avoids first-pass metabolism and GI side effects
• The stratum corneum (outermost skin layer) is the primary barrier, so drugs need adequate lipophilicity to penetrate
• Dosage forms include creams, ointments, transdermal patches, and gels
• Transdermal patches (e.g., nicotine patches, fentanyl patches) deliver drugs systemically at a controlled rate

Rectal Absorption

Drugs administered rectally (as suppositories or enemas) are absorbed through the rectal mucosa.

• Partially avoids first-pass metabolism because the lower rectal veins drain into the systemic circulation rather than the portal vein
• Useful when a patient is vomiting, unconscious, or unable to swallow
• Absorption can be erratic and variable between patients

Pulmonary Absorption

Inhalation delivers drugs directly to the lungs, which offer an enormous surface area (~70 m²) and thin alveolar membranes with rich blood supply.

• Provides very rapid absorption for both local effects (e.g., bronchodilators like salbutamol) and systemic effects (e.g., inhaled anesthetics)
• Dosage forms include metered-dose inhalers (MDIs), dry powder inhalers (DPIs), and nebulizers
• Note: Nasal sprays deliver drugs to the nasal mucosa, not the lungs, and are a separate mucosal route

Mechanisms of Absorption

How a drug crosses biological membranes determines its absorption efficiency. The cell membrane is a phospholipid bilayer, so a drug's physicochemical properties dictate which mechanism dominates.

Passive Diffusion

The most common mechanism for drug absorption. The drug moves down its concentration gradient from high concentration (GI lumen) to low concentration (blood) without energy input.

• Governed by Fick's Law of Diffusion: the rate depends on the concentration gradient, membrane surface area, membrane thickness, and the drug's partition coefficient
• Favors small, lipophilic, unionized molecules
• Not saturable and not subject to competitive inhibition
• Examples: aspirin, ibuprofen, and most small-molecule drugs

Carrier-Mediated Transport

Transport proteins in the membrane move drugs that are too polar or too large for passive diffusion.

• Active transport: Requires ATP. Moves drugs against their concentration gradient. Saturable and subject to competitive inhibition. Examples include intestinal uptake of levodopa via amino acid transporters.
• Facilitated diffusion: Uses carriers but moves drugs down the concentration gradient. No energy required. Also saturable.
• Clinically relevant because drug-drug interactions can occur when two drugs compete for the same transporter

Endocytosis

The cell membrane engulfs the drug molecule, forming an intracellular vesicle.

• Receptor-mediated endocytosis: Specific ligand-receptor binding triggers internalization. Used for targeted delivery of large molecules.
• Pinocytosis (fluid-phase): Nonspecific uptake of dissolved molecules in extracellular fluid.
• Important for absorption of large, polar molecules like peptides, proteins, and nanoparticle-based drug carriers

Paracellular Transport

Drugs pass between cells through tight junctions rather than through them.

• Limited to small, hydrophilic molecules (generally < 200 Da)
• The tight junctions in the intestinal epithelium restrict this pathway, so it accounts for only a small fraction of total absorption
• More significant in "leaky" epithelia (e.g., small intestine) than in "tight" epithelia (e.g., colon, blood-brain barrier)

Factors Affecting Absorption

Drug absorption is rarely a simple process. Multiple variables interact to determine how much drug actually reaches the systemic circulation.

Physicochemical Properties of Drugs

• Lipophilicity: Measured by the partition coefficient (log P). Higher log P generally means better membrane permeability, but extremely lipophilic drugs may have poor aqueous solubility, creating a trade-off.
• Molecular size: Smaller molecules (< 500 Da) are generally absorbed more readily. This is one basis of Lipinski's Rule of Five for predicting oral bioavailability.
• Ionization and pKa: Most drugs are weak acids or bases. The Henderson-Hasselbalch equation predicts the ratio of ionized to unionized drug at a given pH. Only the unionized form readily crosses membranes by passive diffusion.

Formulation Factors

• Dosage form: Solutions are absorbed fastest since the drug is already dissolved. Suspensions, capsules, and tablets require progressively more dissolution time.
• Particle size: Reducing particle size increases surface area, improving dissolution rate (described by the Noyes-Whitney equation).
• Excipients: Disintegrants speed tablet breakup, surfactants improve wetting and solubility, and binders can slow drug release. These choices directly shape the absorption profile.

Physiological Factors

• GI pH: Varies from ~1–3 in the stomach to ~6–7.5 in the intestine. Affects ionization state and solubility of the drug.
• Gastric emptying rate: Faster emptying delivers the drug to the small intestine (the primary absorption site) sooner. Factors like body position, meal composition, and drugs (e.g., metoclopramide speeds emptying; opioids slow it) all play a role.
• GI motility: Increased motility reduces contact time with the absorptive surface, potentially decreasing absorption.
• Splanchnic blood flow: Greater blood flow maintains the concentration gradient across the intestinal membrane, promoting absorption.

Disease States

• GI disorders like Crohn's disease, celiac disease, or short bowel syndrome reduce absorptive surface area or damage the mucosa, impairing absorption
• Hepatic impairment (e.g., cirrhosis) can reduce first-pass metabolism, paradoxically increasing bioavailability for some drugs
• Cardiovascular disease with reduced cardiac output decreases splanchnic blood flow, slowing absorption from the GI tract and IM/SC injection sites

Bioavailability

Bioavailability (F) is the fraction of an administered drug dose that reaches the systemic circulation in unchanged form. It's one of the most important pharmacokinetic parameters because it directly determines how much drug is available to produce a therapeutic effect.

• IV administration has 100% bioavailability by definition (F = 1)
• All other routes have F ≤ 1 due to incomplete absorption and/or first-pass metabolism

Factors Influencing Bioavailability

• First-pass metabolism: After oral absorption, blood from the GI tract flows through the portal vein to the liver before reaching systemic circulation. Drugs extensively metabolized in the liver (e.g., morphine, propranolol) have low oral bioavailability.
• Drug solubility and permeability: The Biopharmaceutics Classification System (BCS) categorizes drugs into four classes based on these two properties, predicting likely bioavailability challenges.
• Efflux transporters: P-glycoprotein (P-gp) in the intestinal epithelium pumps certain drugs back into the GI lumen, reducing absorption.
• Drug-drug interactions: Co-administered drugs can inhibit or induce metabolizing enzymes or transporters, altering bioavailability.

Absolute vs. Relative Bioavailability

• Absolute bioavailability compares a non-IV route to an IV dose of the same drug:

$F_{abs} = \frac{AUC_{oral} / Dose_{oral}}{AUC_{IV} / Dose_{IV}} \times 100\%$

• Relative bioavailability compares two non-IV formulations (e.g., a generic tablet vs. a brand-name tablet):

$F_{rel} = \frac{AUC_{test} / Dose_{test}}{AUC_{reference} / Dose_{reference}} \times 100\%$

Relative bioavailability is the basis for bioequivalence studies used in generic drug approval.

Methods for Determining Bioavailability

1. Plasma concentration-time profiles: Administer the drug, collect blood samples at timed intervals, and measure drug concentration using analytical methods like LC-MS/MS or HPLC.
2. Area under the curve (AUC): Integrate the plasma concentration-time curve. AUC reflects total drug exposure and is the primary metric for bioavailability.
3. Urinary excretion data: Measure cumulative unchanged drug in urine. Useful when blood sampling is impractical, but only works for drugs with significant renal excretion of unchanged compound.

Absorption Kinetics

Absorption kinetics describes how fast and how completely a drug is absorbed over time. These parameters guide decisions about dosing intervals and formulation design.

Zero-Order vs. First-Order Kinetics

• First-order kinetics: The absorption rate is proportional to the amount of drug remaining at the absorption site. Most conventional oral dosage forms follow this pattern. The equation is:

$\frac{dA}{dt} = -k_a \cdot A$

where $k_a$ is the absorption rate constant and $A$ is the amount of drug at the absorption site.

• Zero-order kinetics: The absorption rate is constant regardless of how much drug remains. This occurs with controlled-release formulations, transdermal patches, and IV infusions. The equation is:

$\frac{dA}{dt} = -k_0$

where $k_0$ is the zero-order rate constant.

Absorption Rate Constant ($k_a$)

The absorption rate constant quantifies how quickly a drug is absorbed. It can be determined from the plasma concentration-time curve using the method of residuals (also called feathering or back-extrapolation).

Factors that increase $k_a$: better drug solubility, greater membrane permeability, and larger absorptive surface area.

Absorption Half-Life

The absorption half-life ($t_{1/2,abs}$) is the time for half the drug at the absorption site to be absorbed:

$t_{1/2,abs} = \frac{0.693}{k_a}$

A shorter absorption half-life means faster absorption and quicker onset of action.

Time to Peak Concentration ($T_{max}$)

$T_{max}$ is the time at which the maximum plasma concentration ($C_{max}$) is reached. At this point, the rate of absorption equals the rate of elimination.

• Immediate-release formulations have a shorter $T_{max}$ than sustained-release formulations
• $T_{max}$ depends on both $k_a$ and the elimination rate constant ($k_e$)
• For drugs where rapid onset is critical (e.g., analgesics, anti-anginals), a short $T_{max}$ is desirable

Enhancing Drug Absorption

Many promising drug candidates have poor oral bioavailability. Several strategies have been developed to overcome absorption barriers.

Prodrugs

A prodrug is a pharmacologically inactive compound that is converted to the active drug in vivo through enzymatic or chemical transformation.

• Enalapril is a prodrug of enalaprilat. Enalaprilat itself has poor oral absorption (~10%) due to its polar, ionized structure. Esterifying it to enalapril improves lipophilicity and oral bioavailability to ~60%.
• Valacyclovir is the L-valyl ester prodrug of acyclovir, absorbed via intestinal peptide transporters (PEPT1), increasing oral bioavailability from ~15% to ~55%.
• Prodrug design can also target specific tissues or reduce GI side effects.

Permeation Enhancers

These substances temporarily increase membrane permeability to improve drug absorption.

• Mechanisms: Disrupting the lipid bilayer structure, opening tight junctions between cells, or inhibiting efflux transporters like P-gp
• Examples: Chitosan (opens tight junctions), bile salts (solubilize membrane lipids), fatty acids like sodium caprate
• A key requirement is that the effect must be reversible and non-toxic to the mucosa

Nanoparticle-Based Drug Delivery

Encapsulating drugs in nanoscale carriers (1–1000 nm) can dramatically improve absorption.

• Liposomes: Phospholipid vesicles that encapsulate hydrophilic drugs in their aqueous core or hydrophobic drugs in their lipid bilayer. Example: Doxil (liposomal doxorubicin).
• Polymeric nanoparticles: Biodegradable polymers like PLGA that provide sustained release and protect drugs from degradation.
• Albumin-bound nanoparticles: Example: Abraxane (nab-paclitaxel), which exploits albumin-mediated transcytosis.
• Surface modification with PEG ("PEGylation") extends circulation time; attaching targeting ligands enables cell-specific uptake.

Targeted Drug Delivery Strategies

These approaches aim to concentrate the drug at its site of action while minimizing systemic exposure.

• Antibody-drug conjugates (ADCs): A monoclonal antibody linked to a cytotoxic drug. The antibody binds a specific receptor on target cells (e.g., cancer cells), triggering receptor-mediated endocytosis. Example: Kadcyla (trastuzumab emtansine) for HER2-positive breast cancer.
• pH-sensitive delivery: Exploits pH differences between tissues (e.g., tumor microenvironment is more acidic than normal tissue).
• Magnetic targeting: Drug-loaded magnetic nanoparticles are guided to the target site using an external magnetic field.

Barriers to Absorption

Several biological barriers exist specifically to prevent foreign substances from entering the body. While protective, these barriers also limit drug absorption.

Gastrointestinal Barriers

The GI tract presents a multi-layered defense:

1. Acidic stomach pH (1–3): Degrades acid-labile drugs like proteins, peptides, and certain antibiotics (e.g., erythromycin)
2. Digestive enzymes: Pepsin in the stomach and pancreatic proteases/lipases in the intestine can metabolize drug molecules
3. Mucus layer: A viscous gel covering the epithelium that drugs must diffuse through before reaching absorptive cells
4. Efflux pumps: P-gp and other transporters in the apical membrane actively pump absorbed drugs back into the lumen

Food components can also chelate drugs (e.g., tetracycline binds to calcium in dairy) or compete for transporters.

Blood-Brain Barrier (BBB)

The BBB is formed by brain capillary endothelial cells connected by exceptionally tight junctions, with no fenestrations.

• Only small (< 400–500 Da), lipophilic molecules with fewer than 8–10 hydrogen bonds can cross passively
• Efflux transporters (especially P-gp) are highly expressed, actively pumping many drugs back into the blood
• This is why most anticancer drugs, antibiotics, and large-molecule drugs cannot reach therapeutic concentrations in the brain
• Strategies to cross the BBB include receptor-mediated transcytosis, intranasal delivery, and temporary BBB disruption

Skin Barrier

The stratum corneum is the rate-limiting barrier for transdermal drug delivery.

• Composed of 15–20 layers of dead, flattened keratinocytes embedded in a lipid matrix (often described as a "bricks and mortar" structure)
• Favors absorption of drugs that are small, moderately lipophilic (log P 1–3), and potent at low doses
• Chemical penetration enhancers (e.g., DMSO, azone), iontophoresis, and microneedle arrays are used to overcome this barrier

Placental Barrier

The placenta separates maternal and fetal circulations but is not an absolute barrier.

• Most drugs cross by passive diffusion, so small, lipophilic, unionized drugs transfer most readily
• The placenta also expresses metabolizing enzymes and efflux transporters (P-gp) that provide some fetal protection
• Drug safety in pregnancy is classified by risk categories, and teratogenic drugs (e.g., thalidomide, isotretinoin) must be strictly avoided

Drug-Food Interactions

Food can significantly alter drug absorption, sometimes in clinically dangerous ways. Understanding these interactions is essential for proper dosing instructions.

Types of Drug-Food Interactions

• Pharmacokinetic: Food alters absorption, distribution, metabolism, or elimination. This is the most common type affecting absorption.
• Pharmacodynamic: Food components directly enhance or oppose the drug's effect. Example: Vitamin K in leafy greens antagonizes warfarin's anticoagulant effect.
• Physical/chemical: Food physically binds to or degrades the drug. Example: Tetracycline chelates with calcium in dairy products, forming an insoluble complex that can't be absorbed.

Mechanisms of Drug-Food Interactions

• Altered GI pH: Food buffers gastric acid, raising stomach pH. Drugs requiring acidic conditions for dissolution (e.g., ketoconazole, atazanavir) show reduced absorption when taken with antacids or on a full stomach.
• Delayed gastric emptying: High-fat meals slow gastric emptying, delaying drug delivery to the small intestine. This delays $T_{max}$ but may increase total absorption for some drugs by prolonging contact time.
• Increased splanchnic blood flow: Eating increases blood flow to the GI tract by up to 30%, which can enhance absorption of highly permeable drugs like propranolol.
• Enzyme inhibition: Grapefruit juice inhibits intestinal CYP3A4, increasing bioavailability of drugs like simvastatin, cyclosporine, and felodipine to potentially toxic levels.

Clinical Significance of Drug-Food Interactions

• Warfarin and vitamin K: Patients on warfarin must maintain consistent vitamin K intake. A sudden increase in green vegetables can reduce warfarin's anticoagulant effect, risking clot formation.
• MAO inhibitors and tyramine: Tyramine in aged cheeses, cured meats, and fermented foods is normally metabolized by monoamine oxidase. MAO inhibitors block this, allowing tyramine to accumulate and cause a hypertensive crisis.
• Dosing instructions like "take on an empty stomach" or "take with food" are based directly on these interactions and should be followed carefully.

In Vitro Absorption Studies

In vitro models predict drug absorption before moving to animal or human studies. They're faster, cheaper, and reduce the need for animal testing in early drug development.

Caco-2 Cell Model

The Caco-2 cell line (derived from human colon adenocarcinoma) is the gold standard for in vitro permeability assessment.

• When cultured on permeable filter supports for 21 days, these cells differentiate into polarized monolayers resembling small intestinal enterocytes
• They form tight junctions and express relevant transporters (P-gp, BCRP) and metabolizing enzymes (CYP3A4)
• Drug is added to the apical (donor) side, and appearance on the basolateral (receiver) side is measured to calculate an apparent permeability coefficient ($P_{app}$)
• Bidirectional transport studies (apical-to-basolateral vs. basolateral-to-apical) can identify efflux substrates
• Correlates well with human intestinal absorption for passively absorbed drugs, though it tends to underestimate paracellular transport

Ussing Chamber Technique

This method uses excised intestinal tissue mounted between two half-chambers.

• Each chamber is filled with oxygenated buffer, and drug transport across the tissue is measured over time
• Allows testing of specific intestinal regions (duodenum, jejunum, ileum, colon)
• Can assess the effects of permeation enhancers, pH changes, and formulation components on transport
• Provides information on transport directionality and mechanism, but tissue viability is limited to a few hours

Everted Gut Sac Method

A segment of small intestine is turned inside-out (everted) so the mucosal surface faces outward.

• The sac is filled with buffer, placed in a drug solution, and incubated in oxygenated medium
• Drug appearing inside the sac represents absorbed drug
• More physiologically relevant than cell monolayers because it retains the mucus layer, multiple cell types, and intact architecture
• Useful for studying regional absorption differences and the effects of metabolism on absorption
• Tissue viability limits experiments to approximately 1–2 hours

In Vivo Absorption Studies

In vivo studies provide the most clinically relevant data on drug absorption but are more complex, expensive, and ethically constrained.

Animal Models for Absorption Studies

• Common species: rats (most frequently used for initial screening), dogs (GI physiology closer to humans), rabbits, and pigs
• Allow assessment of absorption, bioavailability, and the effects of formulation variables in a complete biological system
• Species selection should consider anatomical and physiological similarities to humans (e.g., dogs lack a significant cecum; rats are obligate nose-breathers)
• Results require careful extrapolation to humans due to species differences in GI pH, transit time, enzyme expression, and transporter activity

Human Pharmacokinetic Studies

These are the definitive studies for characterizing drug absorption in the target population.

1. Drug is administered to healthy volunteers (Phase I) or patients
2. Blood samples are collected at predetermined time points
3. Plasma drug concentrations are measured using validated analytical methods (LC-MS/MS, HPLC)
4. Key parameters are calculated: $C_{max}$, $T_{max}$, AUC, and bioavailability

These studies can also evaluate the effects of food (fed vs. fasted studies), drug-drug interactions, and special populations (hepatic/renal impairment). They require institutional review board (IRB) approval and informed consent.

Imaging Techniques for Absorption Studies

Non-invasive imaging allows visualization of drug behavior in the intact body.

• Gamma scintigraphy: Radiolabeled dosage forms are tracked through the GI tract using a gamma camera. Useful for studying tablet disintegration, gastric emptying, and regional drug deposition.
• PET (Positron Emission Tomography): Drugs labeled with positron-emitting isotopes (e.g., $^{11}C$, $^{18}F$) can be tracked quantitatively. Particularly valuable for studying BBB penetration and brain distribution of CNS drugs.
• MRI: Can visualize GI fluid volumes, gastric emptying, and intestinal motility without radiation exposure.

These techniques provide real-time, spatially resolved data that complement traditional plasma concentration measurements.