Excipients in Solution Dosages Flashcards

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Excipients

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The constituents of a pharmaceutical form apart from the active substance, serving functions such as solvents, preservatives and stabilisers. They are selected to support drug delivery, stability and patient acceptability while being safe and compatible with the formulation.

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Excipient Specifications

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Specifications define grade, physical form, quantity and functional use for an excipient and may vary by regulatory jurisdiction or monograph. When no monograph exists, one must be developed with documentation on safety and effectiveness.

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Handbook Monographs

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The Handbook of Pharmaceutical Excipients provides monographs on many excipients, including nonproprietary names, empirical formulas, physical properties, incompatibilities, regulatory status and maximum acceptable daily intake. It is a key information source for formulation development and excipient selection.

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Ideal Attributes

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Ideal excipients are chemically and physically stable, low in microbial content, non-toxic and compatible with drug, other excipients and packaging. They should be inexpensive, easy to source and process, palatable, non-sensitising and have low or no odour.

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Excipient Sources

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Excipients originate from natural (e.g., lecithin), semi-synthetic (e.g., cellulose derivatives), and synthetic sources (e.g., poloxamers), as well as mineral oils and polyethylene glycols. Source choice influences purity, functionality and regulatory considerations.

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Solvents

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A solvent is used to dissolve other substances to prepare a solution dosage form, with $H_2O$ being the unique and preferred solvent for many oral formulations. Different grades of water (purified, highly purified, water for injection, sterile WFI) are used depending on biological quality and route of administration.

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Water Grades

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Purified water is for non-sterile preparations, highly purified water for high biological quality needs, water for injection ($WFI$) for parenteral medicines needing pyrogen-free water, and sterile water for injection for parenteral delivery. Grade selection depends on sterility and pyrogen requirements for the route of administration.

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Cosolvents

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Cosolvents are water-miscible organic solvents added to increase the apparent lipophilicity of the solvent and improve drug solubility when $H_2O$ alone is insufficient. Their use raises concerns about toxicity, irritancy, sensitisation, flammability, cost, stability and compatibility with other ingredients and intended route.

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Alcohols

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Common pharmaceutical alcohol cosolvents include ethanol ($C_2H_5OH$), propylene glycol and glycerol; they are used across oral, topical, parenteral and otic routes depending on the agent. Each has distinct properties such as evaporation cooling (ethanol), polyhydric character (propylene glycol, glycerol) and use restrictions in paediatrics.

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Polyethylene Glycols

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Low molecular weight PEGs are polyethoxylates commonly used as cosolvents or carriers in oral and parenteral formulations and have general structure $HOCH_2(CH_2CH_2O)_nCH_2OH$. Their hydrophilicity and viscosity depend on chain length, influencing solubility and formulation handling.

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Other Solvents

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Other useful solvents include fixed (non-volatile) vegetable oils (triglycerides), esters such as ethyl oleate, and dimethyl sulfoxide ($(CH_3)_2SO$) for topical penetration enhancement. Choice depends on viscosity, biocompatibility and intended route (e.g., IM, IV, topical).

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pH Modifiers

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Acidifiers (provide $H^+$) and alkalizers (provide $OH^-$) adjust formulation pH to modify drug solubility and permeability, while buffers resist pH change on dilution or addition of acid/alkali. Common agents include citric acid, acetic acid, ammonium acetate, potassium hydroxide and sodium bicarbonate ($NaHCO_3$).

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Solubilisers

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Solubilisers increase the apparent aqueous solubility of poorly water-soluble drugs so doses are contained in manageable volumes and can cross membranes. Examples include cyclodextrins, polyoxyethylene castor oil, poloxamers and polysorbates.

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Wetting Agents

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Wetting agents reduce surface tension to permit hydrophobic powders to displace air and become wetted by water, facilitating processing and dissolution. Common wetting agents include polysorbates, sodium lauryl sulphate and polyoxyl-40 stearate.

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Levigating Agents

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Levigating agents are liquids or semi-solids used to aid reduction of particle size during grinding, improving homogeneity and texture in semisolid preparations. Examples include mineral oil, glycerol and propylene glycol.

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Clarifiers

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Clarifiers are filter aids that adsorb undissolved solids to assist removal by filtration, producing a clear solution suitable for tinctures, mouthwashes and ophthalmic products. Examples include certain silica-based adsorbents and crospovidone.

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Humectants

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Humectants are added to liquid products to prevent evaporation and drying by interacting with the solvent to lower vapour pressure, a colligative effect. Typical humectants include glycerol, propylene glycol, sorbitol and $PEG300$, and they help minimise crystallisation and cap-locking.

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Bulking Agents

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Bulking agents for freeze-drying (lyophilisation) protect drugs from freeze concentration and stabilise structure during sublimation and reconstitution. Common bulking agents include trehalose, mannitol and glycine.

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Preservation

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Preservation involves adding agents to multidose aqueous preparations to retard microbial proliferation and extend shelf-life; activity depends on formulation ingredients and container. Not all formulations are self-preserving, and an assumption of inherent antimicrobial activity is unsafe.

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Preservative Criteria

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Ideal preservatives are non-toxic, non-irritant, non-sensitising, broadly active over a wide $pH$ range, chemically stable, soluble and compatible with other ingredients and containers. No single preservative satisfies all criteria, so selection balances spectrum, stability and compatibility.

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Key Preservatives

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Common preservatives include parabens (e.g., methyl, propyl), benzalkonium chloride, benzoic acid, benzyl alcohol and sorbic acid, with uses varying by route such as oral, parenteral and ophthalmic. Selection depends on spectrum, formulation $pH$ and compatibility.

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Quaternary Ammonium

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Quaternary ammonium compounds like benzalkonium chloride are cationic preservatives effective over wide $pH$ ranges and perturb bacterial membranes. They are incompatible with anionic substances and may irritate skin and mucous membranes.

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Parabens

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Parabens are alkyl esters of $p$-hydroxybenzoic acid (methyl, ethyl, propyl, butyl) whose antimicrobial activity increases with longer alkyl chains while solubility decreases. They are often used as mixtures (e.g., methyl:propyl 2:1) for synergistic effects and are active roughly between $pH\text{ }4$ to $9$ in their non-ionised form.

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Benzoic Acid

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Benzoic acid has antibacterial and antifungal activity primarily in its unionised form and is therefore effective at acidic conditions (active below about $pH\text{ }4$). It is only sparingly soluble in water and carries potential for allergic reactions and formulation incompatibilities.

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Sorbic Acid

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Sorbic acid exhibits antibacterial and antifungal properties, optimally active around $pH\text{ }4.5$ and largely inactive above $pH\text{ }6$. It synergises with other preservatives or glycols but is sensitive to oxidation producing potentially toxic byproducts and can be detoxified by moulds via decarboxylation.

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Preservative Efficacy Test

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The European Pharmacopoeial preservative efficacy test (EP test) challenges the final product with a microbial inoculum (typically $10^5$-$10^6$ cells) and monitors survival at specified timepoints (e.g., $t=0$, 6 h, 24 h, 48 h, 7, 14, 28 days). Acceptance criteria depend on formulation type with stringent requirements for injectables (e.g., a 1000-fold decrease after 24 h).

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Antioxidants

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Antioxidants are added to retard oxidative processes that can degrade drugs or excipients, processes catalysed by light, heat, $pH$, trace metals or peroxides. Oxidation can be seen as an increase in carbon-to-oxygen ratio or decrease in carbon-to-hydrogen ratio and is mitigated by various antioxidant classes.

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Antioxidant Types

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True antioxidants rapidly quench free radicals to block propagation, reducing antioxidants are sacrificial agents with lower redox potential than the drug, and synergists enhance antioxidant activity often by chelation. Examples include ascorbic acid (reducing), $EDTA$ (chelating/synergy) and $\alpha$-tocopherol (true antioxidant).

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Flavours and Taste

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Flavours, sweeteners and colours optimise organoleptic properties to improve patient compliance, especially in oral liquids. Taste preferences vary by age (children prefer sweet) and chemical properties correlate with taste (e.g., sourness due to $H^+$, sweetness often from polyhydroxyl compounds).

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Taste Masking

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Taste masking strategies include blending (matching a flavour profile), overshadowing (stronger flavour), physical methods (suspensions, emulsions, viscosifiers), chemical methods (adsorption, complexation) and physiological methods (cooling agents or sialagogues). The goal is rapid flavour identity, full development, acceptable mouthfeel and minimal aftertaste.

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Sweeteners

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Sweeteners impart sweetness and are chosen for sweetness potency, caloric content and stability; sucrose is traditional, aspartame is high-potency but pH-sensitive, sorbitol provides bulk and stability, and saccharin is very potent but has a strong aftertaste. Selection depends on target population (e.g., diabetics) and formulation constraints.

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Sialagogues

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Sialagogues such as citric, fumaric and tartaric acids stimulate saliva production to reduce residence time of undesirable tastes in the mouth. They can be used as a physiological taste-masking approach in oral formulations.

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Viscosity Enhancers

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Viscosity enhancers are macromolecular colloids that increase resistance to flow to improve dose uniformity, mouthfeel and contact time with biological surfaces. Examples include carbomers, carboxymethylcellulose, acacia, poloxamer and povidone.

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Colouring

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Colourants (dyes for liquids, lakes for solids) are used to match flavour, improve identification and mask minor colour deterioration. They may be mineral, natural or synthetic and raise acceptability concerns if derived from animals or associated with pharmacological effects.