DDS Test 1

Overview of Oral Drug Delivery and Dosage Forms

Oral drug delivery is a central route of administration in pharmaceutical science. The gastrointestinal (GI) tract provides a large surface area for drug absorption and is generally well-accepted by patients due to ease of use. However, it faces challenges including high metabolic activity, extreme pH levels, and variability in intestinal motility. Common oral dosage forms include tablets, capsules, solutions, and suspensions. The process of drug absorption follows a specific sequence: disintegration of the dosage form into smaller particles, dissolution of these drug particles into the gastrointestinal fluids, and finally, absorption into the systemic circulation. For drugs with poor aqueous solubility, the slowest step, known as the rate-limiting step, is typically the dissolution rate. Bioavailability is defined as the fraction or percentage of the administered dose that reaches the systemic circulation intact. For example, a $100\,mg$ dose may only result in $30\,mg$ of active drug in the bloodstream, indicating a $30\% \text{ bioavailability}$.

The Noyes-Whitney Equation and Dissolution Kinetics

The dissolution of solid drugs is described mathematically by the Noyes-Whitney equation, which expresses the dissolution rate as directly proportional to several factors and indirectly proportional to the thickness of the diffusion layer. The rate is expressed as:

$\frac{dC}{dt} = \frac{D \times S \times (C_s - C)}{h}$

Where $D$ is the diffusion coefficient, which increases with higher temperature, lower solvent viscosity, and smaller molecular size of the drug. $S$ represents the surface area, which can be improved via particle size reduction (micronization). The term $(C_s - C)$ is the concentration gradient across the diffusion layer, where $C_s$ is the solubility of the drug in the diffusion layer and $C$ is the concentration in the bulk fluid. Improving dissolution often involves increasing $C_s$ by using amorphous forms of drugs or reducing $C$ through fluid intake or rapid absorption. Finally, $h$ represents the thickness of the diffusion layer, which can be decreased through mechanical stirring (e.g., intestinal motility).

Physiological and Bioavailability Factors

Several physiological factors significantly influence the oral bioavailability of drugs. Gastrointestinal motility, specifically the gastric emptying rate, affects how quickly the drug reaches the small intestine, which is the primary site for absorption. Factors affecting this include postural position and food composition. The pH of the GI tract affects drug solubility and stability; for instance, weak acids are more unionized and less soluble in the low pH of the stomach ($pH\,1-3$), while weak bases are more ionized and soluble in that environment. Digestive enzymes (like CYP450) and bacteria in the colon can also degrade drugs. Furthermore, high GI viscosity caused by food can reduce the rate of drug diffusion. Interactions can be direct (chemical binding like tetracylines with aluminum antacids or iron) or indirect (alterations in pH or gastric emptying).

Strategies to Enhance Drug Solubility and Bioavailability

Pharmaceutical scientists use various strategies to address low drug solubility. Co-solvents such as propylene glycol, ethanol, and glycerin are added to aqueous systems to reduce solvent polarity and increase solubility. Salt formation (e.g., diclofenac sodium) is used to create a more soluble version of the drug. Surfactants like sodium lauryl sulfate ($SLS$) or polysorbate $80$ reduce surface tension and form micelles to solubilize hydrophobic drugs. Cyclodextrins provide a molecular "hideout" for drugs to improve solubility. Particle size reduction increases the effective surface area ($S$) available for dissolution. Polymorphism also plays a role, with amorphous forms typically having higher solubility than crystalline or metastable forms. Amorphous solid dispersions involve dispersing a drug in a hydrophilic polymer to enhance release in aqueous media. Conversely, the common ion effect can reduce solubility; adding more ions (common ions) to a system shifts the equilibrium and causes precipitation of the drug.

Physicochemical Parameters: Dissociation and Partitioning

The degree of ionization is calculated using the Henderson-Hasselbalch equation. Weak acids like Aspirin ($pK_a = 3.5$) remain predominantly unionized in the stomach, while weak bases like Codeine ($pK_a = 8$) remain unionized in the more neutral environment of the intestine ($pH\,6-8$). The partition coefficient ($P$) is an indicator of lipid solubility, defined as:

$P = \frac{[drug]_{\text{organic phase}}}{[drug]_{\text{aqueous phase}}}$

A Log\,P > 0 indicates the drug is lipid-soluble, while a Log\,P < 0 indicates water solubility. The ideal range for oral drugs is typically 3 < Log\,P < 6. Prodrugs, such as Enalapril or Pivampicillin, are often designed to increase lipid solubility; for example, the bioavailability of ampicillin ($32-55\%$) increases to $87-94\%$ when administered as the prodrug pivampicillin. Lipinski's Rule of Five provides criteria for orally active drugs: molecular weight less than $500\,Da$, a maximum of $5\text{ hydrogen bond donors}$, a maximum of $10\text{ hydrogen bond acceptors}$, and a $Log\,P$ not greater than $5$.

Principles of Dosage Form Design

Dosage form design aims to create a formulation that ensures a predictable therapeutic response and can be manufactured at a large scale with consistent quality. Key principles include safety, accuracy, stability, patient acceptability, and targeted action. A drug delivery system differs from a dosage form in that it releases medicine in a controlled way (rate, time, and location). For example, the Adalat GITS system uses an osmotic "push-pull" mechanism. This bilayer tablet contains a polymeric "push compartment" and a drug reservoir; as water enters through a semi-permeable membrane, the osmotic layer swells and forces the drug (nifedipine) out through a laser-drilled hole, providing even administration over a $24\text{-hour}$ period. General considerations for design include therapeutic matters (e.g., chronic vs. emergency disease), the age of the patient (e.g., pediatric vs. comatose), and the drug's inherent chemical factors.

Excipients in Oral Formulations

Dosage forms comprise the active pharmaceutical ingredient (API) and various excipients. Diluents (e.g., lactose, sucrose, mannitol) increase the bulk of the dosage form. A historical case in Australia in $1960$ showed that changing the phenytoin diluent from calcium sulphate to lactose led to toxicity because calcium sulphate previously formed an insoluble complex with phenytoin, and the switch to lactose significantly increased the drug's bioavailability. Surfactants ($SLS$, lecithin) act as emulsifying, solubilizing, or wetting agents ($HLB\,7-9$). Lubricants (e.g., magnesium stearate) reduce friction during manufacturing but can reduce dissolution rates if used in excess due to their hydrophobic nature. Disintegrants (e.g., starch, sodium starch glycolate) help break capsules and tablets into primary particles by swelling. Viscosity-enhancing agents (e.g., xanthan gum, HPMC) control pouring and sedimentation in liquid forms but may slow drug absorption by reducing molecular movement or forming complexes.

Liquid and Semi-Solid Dosage Forms: Solutions, Emulsions, and Ointments

Pharmaceutical solutions are homogenous one-phase systems ($solute + solvent$). Aqueous solutions use purified water or water for injection, while non-aqueous solutions (oily liquids) are used for drugs unstable in water or for depot therapy (e.g., testosterone propionate IM injections). Emulsions consist of two immiscible liquids stabilized by an emulsifying agent. They can be oil-in-water ($O/W$), which are non-greasy, or water-in-oil ($W/O$), which are greasy. The Hydrophile-Lipophile Balance ($HLB$) system, ranging from $1$ to $20$, dictates the type of emulsion: high $HLB$ agents form $O/W$ emulsions while low $HLB$ agents form $W/O$ ones. Instabilities in emulsions include flocculation (loose clusters), creaming (aggregation at top/bottom), coalescence (cracking/irreversible), and phase inversion. Ointments are greasy semi-solid preparations for external use. Their bases include oleaginous (hydrocarbon) bases like petrolatum, absorption bases which can absorb water, emulsion bases (easily washable), and water-soluble bases like Polyethylene Glycol ($PEG$).

Solid Dosage Forms: Tablets, Capsules, and Powders

Tablets are obtained by compressing uniform volumes of particles. Types include immediate-release (conventional), chewable, effervescent (involving a citric acid and carbonate reaction to release $CO_2$), and layered tablets (to separate incompatible drugs). Tablet coatings protect the drug, mask taste, or control release (e.g., enteric coating). Capsules are solid preparations with hard or soft gelatin shells. Hard gelatin capsules typically hold dry solids, while softgels can contain solutions or suspensions. Gelatin is favored for its safety and flexibility, though non-gelatin alternatives like $HPMC$ are available for vegetarians. Powders and granules are finely divided solids. Their flow is characterized by the Carr's Index ($CI$), Hausner Ratio ($HR$), and Angle of Repose. A CI < 10\% or HR < 1.25 indicates excellent flow, while an angle of repose close to $25^\circ$ is ideal. Poor flow can cause manufacturing issues like capping (tablet top separation) or lamination (layer separation).

Suspensions and Stability

Suspensions are preparations containing finely divided drug particles (coarse > 1\,\mu m, colloidal < 1\,\mu m) distributed in a vehicle where the drug has minimal solubility. A good suspension remains uniform for accurate dosing after shaking and redisperses easily. Wetting agents are crucial to displace air on the particle surface to allow for dispersion. Suspending agents like tragacanth or methyl cellulose increase viscosity to prevent rapid settling. Deflocculated suspensions settle slowly but can form a hard "cake" that is difficult to redisperse. Flocculated suspensions use flocculating agents (electrolytes or polymers) to form loose clusters that settle faster but are easily redispersed with gentle shaking.