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Excipients Used in Formulation of Liquid Dosage Forms

Along with the active drug substance, liquid dosage forms contain excipients. Vehicle/solvent, Co-solvents, Surfactants, Viscosity/suspending agents.
Along with the active drug substance, liquid dosage forms contain excipients. These serve a variety of pharmaceutical purposes. The amount of these excipients is usually greater than the amount of the drug substance since the drug substance comprises the bulk of the formulation. To ensure that liquid dosage forms are made of chemically and physically compatible components, all of the excipients that are used in the preparation should be compatible with every other component.

These excipients are included in the formulation of liquid dosage forms for pharmaceuticals:

Vehicle/solvent

The vehicle is an important component of liquid pharmaceutical formulations, as it is a platform in which excipients and drugs are dissolved or dispersed. Hence, when bonds break, effective charges on ions decrease, which increases forces of attraction between solutes and solvents, which are ultimately greater than that between solutes and solvents; There are several ways to formulate liquid dosage forms using water, polyhydric alcohols, hydro-alcoholic solutions, and buffers, as well as oils or organic oily bases. Examples of vessels include vegetable oils or mineral oils, organic oily bases, and emulsified bases. Physicochemical properties of the active pharmaceutical ingredient (API) determine what kind of vehicle is used and the formulation's intended use.
  • Water can be used as solvents
  • Alcohol can also be used as solvents
  • Glycerine, USP (glycerol) can be used as solvents
  • Propyl glycol is also an option for being used as solvents
  • Polyethylene glycol is also a good vehicle for liquid dosage forms

Co-solvents

Essentially, co-solvents are designed to increase the water solubility of drugs that do not contain ionic groups and their water solubility cannot be increased by adjusting the pH. It is possible for them to function because aqueous solutions and hydrophobic solutes have decreasing pressures.

Hydrogen bond donors and acceptors present in co-solvents enable them to bind to water and be miscible with it. It is important to consider several factors when choosing a co-solvent, such as a solubility and stability of the drug substance in the vehicle, as well as the vehicle's toxic properties. Water-miscible organic liquids however tend to be toxic and are rarely found in pharmaceutical solutions.

Co-solvents have a specified range of acceptable concentrations that cannot be exceeded without causing biological damage. When co-solvents are used in parenteral formulations, they can generally cause embolism and necrosis at the injection site due to the uncontrolled precipitation of drug substances after dilution in aqueous or biological media. In addition to in vitro and in vivo models, in vitro and in vivo studies have been conducted to evaluate excipients and co-solvents. The low molecular weight PEGs, glycerol, and propylene glycol are examples of excipients used as cosolvents.

Surfactants

Microparticles in aqueous media can be described as dynamic aggregates with defined polar (hydrophilic) and nonpolar (hydrophobic) regions, called surfactants or surface-active agents. It's possible for drugs that are non-polar to partition into soluble micelles.

Surfactants vary considerably with the nature of the polar area, especially sodium dodecyl sulfate, cationic (trialkyl ammonium), zwitterionic (glycine and proteins), and non-ionic (polyethylene glycol). Surfactants come in many different types in addition to anionic and non-ionic types.

Non-ionic surfactants are generally considered to be more suitable in pharmaceutical applications due to their lower toxicity as well as because their shells exhibit stealth properties without absorbing through the reticular endothelial system (RES). Therefore, their lifetime in the bloodstream is prolonged.

A higher concentration of surfactants is generally necessary when enhancing drug solubility due to micelles, which cause solubilization. A critical micelle concentration is a concentration that causes microcapsules to form. A normal micelle will either be spherical, cylindrical, or lamellar in shape, depending on the surfactant concentration.

Preservatives
Formulations are protected from microbial contamination with preservatives, chemical compounds added to them. The best preservatives are those that do not contain harmful substances.
  • Microorganisms of all types can be killed at low concentrations
  • API, other excipients, and the container system soluble, non-sensitizing, and non-toxic
  • that prevents the substance from oxidizing.
When liquid dosage forms are manufactured in an aqueous environment, there is a significant risk of contamination by microorganisms. As such, the use of preservatives as a method of prevention upon production and storage becomes necessary in such cases. There is a rising concern about the bioactivity and adverse effects of these excipients, so it would be ideal if no preservatives were used in formulations. Unfortunately, most formulations must include preservatives to ensure no microbial growth occurs.

The majority of preservatives are acidic or non-acidic and act bacteriostatically rather than bactericidal. The acidic types include phenol, benzoic acid, boric acid, chlorocresol, 9-phenyl phenol, alkyl esters of para hydroxybenzoic acid, and sorbic acid, as well as their salts. In addition to chlorobutanol and benzyl alcohol, beta-phenylethyl alcohol can also be used as preservatives. The need for preservatives is generally not recommended at alkaline pH values since it is believed that microbial growth will be minimal.

The need for preservatives is generally not recommended at alkaline pH values since it is believed that microbial growth will be minimal. Additionally, several parameters should be evaluated during the formulation development process in connection with the API, other excipients, and container system, as well as the antimicrobial potential of the excipient.

Viscosity/suspending agents

Excipients such as viscosity modifiers and suspending agents, which act as energy barriers and minimize particle attachment and aggregation are known as viscosity modifiers and suspending agents, respectively. When making a pharmaceutical suspension, selecting a suitable suspending agent is one of the most critical steps. Choosing a suspension agent, for instance, involves taking into account several factors, such as rheological properties, suspension ability, pH stability, chemical compatibility with the drug, and hydration time.

In addition to polymers synthesized (such as carbomers, polyvinyl pyrrolidone, polyvinyl alcohol, and poloxamers), other compounds are reported (such as colloidal silicon dioxide and silicates). Combining these excipients is common in many cases.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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