Parenteral drug products are the dosage forms intended for administration by a route that does not involve the gastrointestinal (GI) tract (thus, parenteral). Most of the parenteral drug products are injectable dosage forms that are intended for administration by injection using a syringe and a needle. Parenteral dosage forms are preferred for one or more of the following reasons:
• Low oral bioavailability and/or high variability in oral drug absorption.
• Instability of the drug in the GI tract. For example, most protein drugs are highly unstable.
• Rapid onset of drug action is desired.
• Ability to immediately stop drug administration is important. For example, most emergency room medications and anesthetics.
• High degree of flexibility in dosage adjustment with or without real-time patient physiological response is needed. For example, emergency medications such as analgesics, anticancer drugs, and fertility medications.
Many drugs are available only as parenteral dosage forms. These include most protein and peptide drugs, some antibiotics, heparin, lidocaine, protamine, glucagon, and many anticancer compounds.
Most injections are designed for administration into a vein (intravenous, IV), into a muscle (intramuscular, IM), into the skin (intradermal, ID), or under the skin (subcutaneous, SC). Nevertheless, drugs may be administered into almost any organs or area in the body, including the joints (intraarticular), joint fluid area (intrasynovial), spinal column (intraspinal), spinal fluid (intrathecal), arteries (intraarterial), and in the heart (intracardiac). In addition, parenteral routes of administration include dosage forms such as sublingual tablets, transdermal patches, and inhalers.
The IV administration provides immediate access of the drug to the systemic circulation, resulting in the rapid onset of drug action. Depending on the rate of drug administration, IV injections could be a bolus or an infusion. A bolus means the drug is injected into the vein over a short period of time. A bolus is used to administer a relatively small volume and is often written as IV push (IVP).
An infusion refers to the introduction of larger volumes (100–1000 mL) of the drug over a longer period of time. A continuous infusion is used to administer a large volume of drug at a constant rate. Intermittent infusions are used to administer a relatively small volume of drug over a specified amount of time at specified intervals.
IV infusion can be administered through peripheral veins, typically in the forearm or the peripherally inserted central catheter. The commonly administered IV infusion products include Lactated Ringers Injection USP; Sodium Chloride Injection USP (0.9%), which replenish fluids and electrolytes; and Dextrose Injection USP (5%), which provides fluid plus nutrition; and various combinations of dextrose and saline. Other solutions of essential amino acids or lipid emulsions are also used as infusions.
IM injections of drugs into the striated muscle fibers that lie beneath the SC layer provide effects that are less rapid but generally longer lasting than those obtained from IV administration. Aqueous or oleaginous solutions or suspensions of drugs may be administered intramuscularly. Drugs in aqueous solution are absorbed more rapidly than those in oleaginous preparations or in suspensions. An IM medication is injected deep into a large muscle mass, such as the upper arm, thigh, or buttocks. Up to 2 mL of the drug may be injected into the upper arm and 5 mL in the gluteal medial muscle of each buttock.
Numerous dosage forms are administered through this route of administration, including solutions (aqueous- or oil-based), emulsions (o/w or w/o), suspensions (aqueous- or oil-based), colloidal suspensions, and re-constitutable powders. Slow drug absorption leading to a sustained-release (SR) effect can be achieved with highly insoluble drugs or formulations that are oleaginous or particulate. IM injections are often painful and nonreversible, that is, the administered drug cannot be withdrawn if needed. Antibiotics are often administered by this route.
The SC route is used for small volume injections, typically 1 mL or less. SC injections are administered beneath the surface of the skin, between the dermis and muscle. Medications administered by this route are slowly absorbed and consequently have a slower onset of action than medications given by IV or IM routes. Drugs often given by this route include epinephrine, insulin, heparin, scopolamine, and vaccines. Small injection volume often puts limitations on the drugs that can be administered by this route. For example, high dose drugs that tend to become highly viscous at high concentrations, such as most globular proteins, are usually difficult to formulate as subcutaneous injectable dosage forms.
Intradermal administration involves injection just beneath the epidermis, within the dermal or skin layers. The usual site for intradermal injection is the anterior forearm. The volume of solution that can be administered intradermally is limited to 0.1 mL. The onset of action and the rate of absorption of medication from this route are slow. This route is used for diagnostic agents, desensitization, testing for potential allergies, or immunization.
Intrathecal route involves drug administration into the cerebrospinal fluid (CSF). This route is needed if CSF is the desired site of drug action because most drugs do not reach the CSF from the systemic circulation. Drugs administered intrathecally include antineoplastics, antibiotics, anti-inflammatory, and diagnostic agents.
An intraarticular injection is made into the synovial cavity of a joint, usually to obtain a local therapeutic effect. For example, an intraarticular injection of a corticosteroid provides an anti- inflammatory action in an arthritic joint.
An intra-arterial injection is made directly into an artery that has been surgically isolated if it is necessary to deliver a high concentration of drug to a diseased organ, such as kidney, with minimal distribution to other systemic locations.
An intraocular injection is made directly into the eye. For example, an injection into the vitreous humor provides access of drug to the rear regions of the eye, such as the retina, which does not receive high drug concentration on topical administration.
Factors affecting selection of route
Selection of a parenteral route of administration for a new therapeutic moiety depends on several considerations, such as
• Desired rate of onset of action: IV route provides the most rapid onset of action, whereas the SC, IM, and IP routes have slower rate of drug absorption into the systemic circulation. SC route is often preferred for SR dosage forms when slow drug absorption over a prolonged period is desired.
• Location of drug action: Intraarterial injections are preferred for localized drug action in an organ, whereas IP route is preferred if drug action is desired in the lymphatic system.
• Tissue irritability: Injection of an irritant drug is likely to be more painful by the IM than the SC route due to higher blood flow and sensory innervations in the muscles.
• Injection volume: The volume of drug injected is lower for SC than for IM or IV routes. In certain cases, formulation of low volume injections is not feasible, especially for protein drugs with high doses.