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About Author:
Mahek Goel

Shri Baba Mastnath Institute of Pharmaceutical Science & Research
Asthal Bohar, Rohtak, Haryana (124001)

Parenteral drug delivery systems are the preparations that are given other than oral route. (Para-outside, enteric–intestine). Parenteral drug delivery systems are most preferred drug delivery systems as they meet many benefits over other dosage forms in many cases such as unconsciousness, nausea, in emergency clinical episodes. The Parenteral administration route is the most common and efficient for delivery of active drug substances with poor bio-availability and the drugs with a narrow therapeutic index. But parenteral route offers rapid onset of action with rapid declines of systemic drug level. For the sake of effective treatment it is often desirable to maintain systemic drug levels within the therapeutically effective concentration range for as long as treatment calls for. It requires frequent injection, which ultimately leads to patient discomfort. For this reason, drug delivery system which can reduce total number of injection throughout the effective treatment, improve patient compliance as well as pharmacoeconomic. These biodegradable injectable drug delivery system offer attractive opportunities for protein delivery and could possibly extend patent life of protein drugs.Parenteral drug delivery system seeks to optimize therapeutic index by providing immediate drug to the systemic pool in required quantity to treat– cardiac attacks, respiratory attacks. This article explores various prolonged release parenteral drug delivery system and their strategies of preparation, their potential benefits/drawbacks and in-vitro testing methods.


The Parenteral administration route is the most effective and common form of delivery for active drug substances with metabolic bio-availabilities drug for which the bio-availability in limited by high first pass metabolism effect of other physicochemical limitation and for drugs with a narrow therapeutic index.

For this reason, whatever drug delivery technology that can reduce the total number of injection throughout the drug therapy period will be truly advantageous not only in terms of compliance, but also for potential to improve the quality of the therapy. Such reduction in frequency of drug dosing is achieved, in practice, by the use of specific formulation technologies that guarantee that the release of the active drug substance happens in a slow and predictable manner.

For several drugs, depending on the dose, it may be possible to reduce the injection frequency from daily to once or twice monthly or even less frequently. In addition to improving patient comfort, less frequent injection of drugs in the form of depot formulation smoothes out the plasma concentration time profiles by eliminating the peaks and valleys. Such smoothing out of the plasma profiles has the potential to not only boost the therapeutic benefit but also to reduce unwanted events and side effects.

The release can either be continuous or pulsatile depending on the structure of the device and the polymer characteristics, continuous release profiles are suitable to generate on ‘infusion like’ plasma level time profile in the systemic circulation without the necessity of hospitalization.

Properties of ideal parenteral controlled drug delivery system:
•          Safe from accidental release
•          Simple to administer and remove
•          Inert
•          Biocompatible
•          Mechanically strong
•          Comfortable for the patient
•          Capable of achieving high drug loading
•          Readily processable
•          Easy to fabricate and sterilize
•          Free of leachable impurities

Advantage and Disadvantage:

(a)       Convenience

(b)       Compliance Potential for controlled release

(c)   Avoiding the peak (risk of toxicity) at troughs (risk of ineffectiveness of conventional therapy)
(i)          Reducing the dosing frequency.
(ii)         Increasing patient compliance.

(d)       Improved drug delivery

(e)       Flexibility

2. Disadvantage:
(a)      Invasive
(b)      Danger of device failure
(c)      Limited to potent drug
(d)      Commercial disadvantage

Approaches used for the development of Parenteral controlled release formulations:
• Use of viscous, water miscible vehicles – aq. Solution of gelatin.
• Use of water immiscible vehicles – vegetable oils + aluminium monosterate
• Formation of thixotropic suspension
• Preparation of water insoluble drug derivatives – salts, complexes and esters.
• Dispersion in polymeric microspheres and microcapsules like lactide–glycolide     homopolymers/ co-polymers.
• Co-administration of vasoconstrictors.

Polymers used in Parenteral controlled drug delivery system:
Generally,Biodegradable polymers are used for the preparation of parenteral controlled drug delivery system as it get degraded in the body and hence doesnot require removal from the body.

Classification of Biodegradable Polymers Biodegradable polymer may be classified based on the mechanism of release of the drug entrapped in it: 
Natural -
 albumin starch, dextran, gelatin, fibrinogen, hemoglobin.  

Synthetic - -cynoacrylates), poly ethyl--poly (alkyl  cynoacrylates, poly amides. Nylon 6-10 nylon-cynoacrylates, poly butyl - 6-6, poly acryl amides, poly amino acid, poly urethane. Aliphatic poly esters are poly (lactic acid) poly lactide - co glycolide) poly glycolic acid, poly caprolactone, polydihydroxy butyrate, poly hydroxy butyrate co-valently cross linked protein, hydrogel.

Biodegradable polymers investigated for controlled drug delivery are Poly lactide / poly glycolide polymers, Poly anhydrides, Poly caprolactone, Poly orthoesters, Psuedo polyamino acid, Poly phosphazenes, Natural polymers

Desirable characteristics of an ideal Parenteral drug carrier

  1. Versatility in that carrier can deliver a variety of agents.
  2. High capacity to carry a sufficient quantity of drug per unit carrier to release therapeutic concentration to the target site without excessively loading host with the carrier.
  3. Restricting drug distribution to the desired target tissue.
  4. Uniform distribution within the capillary vasculature of the target tissue.
  5. Affording drug ready access to the parenchyma of target tissue.
  6. Restricting drug activity at the target site over a prolonged period.
  7. Minimizing systemic drug release during intravascular transit.
  8. Protecting drug from inactivation by plasma enzymes.
  9. Being biocompatible and minimally antigenic.
  10. Undergoing biologic degradation with prompt elimination and minimal toxicity of the breakdown products.

Major routes of Parenteral administration

  • Intravenous (into a vein), e.g. many drugs, total parenteral nutrition (TPN)
  • Intraarterial (into an artery), e.g. vasodilator drugs in the treatment of vasospasm and thrombolytic drugs for treatment of embolism
  • Intramuscular (into a muscle), e.g. many vaccines, antibiotics, and long-acting psychoactive agents
  • Intracardiac (into the heart), e.g. adrenalin during cardiopulmonary resuscitation (not commonly performed anymore)
  • Subcutaneous (under the skin), e.g. insulin
  • Intraosseous infusion (into the bone marrow) is, in effect, an indirect intravenous access because the bone marrow drains directly into the venous system. This route is occasionally used for drugs and fluids in emergency medicine and pediatrics when intravenous access is difficult
  • Intradermal, (into the skin itself) is used for skin testing some allergens, and also for tattoos
  • Intraperitoneal, (into the peritoneum) is predominantly used in veterinary medicine and animal testing for the administration of systemic drugs and fluids due to the ease of administration compared with other parenteral methods.
  • Epidural (synonym: peridural) (injection or infusion into the epidural space), e.g. epidural anesthesia
  • Intrathecal (injection or infusion into the cerebrospinal fluid), e.g. antibiotics, spinal anesthesia.

Biopharmaceutic considerations
• For a drug administered intramuscularly or subcutaneously to reach the site of action to execute therapeutic activity, it must first be released from its formulation, transported from the injection site into the systemic circulation, and then delivered to the target tissue.

• For a drug administered parenteral in the form of a suspension in either aqueous or oleaginous vehicle the rate of delivery and the extent of availability of the drug to the site of action is frequently found to be controlled by the slowest (rate- limiting) step in the pharmacokinetic sequence as shown below.


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