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Renin Angiotensin Mechanism.

Formulation Approaches of TDDS.

 

  • TDDS can be formulated by different ways as follows,

    • Polymer membrane permeation controlled TDDS.

    • Polymer matrix diffusion controlled TDDS.

    • Adhesive Dispersion – Type Systems.

    • Microreservoir dissolution controlled TDDS.

  1. Polymer membrane permeation controlled TDDS:

  • A polymeric membrane that controls flow rate is positioned between a backing laminate that is drug-impermeable and a drug reservoir. 

  • The drug is evenly distributed throughout the drug reservoir compartment in a solid polymeric matrix (like polyisobutylene) and suspended in a viscous, non-leachable liquid medium (like silicon fluid) to create a paste-like suspension. 

  • A polymeric membrane that controls rate can be either microporous or nonporous, such as ethylene-vinyl acetate copolymer. 

  • Estraderm (used twice weekly to treat postmenopausal syndrome) and Duragesic (used to manage chronic pain for 72 hours) are two examples of this type of patch. 

  • The intrinsic rate of drug release from this type of drug delivery system is defined by 

    • {dq/dt}=Cr/1/Pm+1/Pa. 

  • Where, 

    • Cr = Concentration of drug in the drug reservoir. 

    • Pa= Permeation Coefficient of adhesive layer. 

    • Pm= Permeation Coefficient of rate controlling membrane. 

  1. Polymer matrix diffusion controlled TDD system:

  • This method involves uniformly dispersing drug particles in a hydrophilic (or lipophilic) polymer matrix to create the drug reservoir. 

  • A disc with a specific surface area and controlled thickness is then formed from the resulting polymer matrix. 

  • The medicated disc is then moulded onto an occlusive base plate in a compartment made of a drug impermeable backing. 

  • The film is then covered in adhesive polymer around its perimeter. 

  • Examples include the 0.5g/cm2 daily dose of nitro-glycerine-releasing transdermal therapeutic system for angina pectoris.

  • Rate of drug release in this system is given by the equation 

    • dq/dt = {ACpDp/2t}1/2

  • Where, 

    • A= Initial drug loading dose dispersed in polymer matrix 

    • Cp = Solubility of drug in Polymer 

    • Dp = Diffusivity of drug in Polymer since Cp is equal to Cr. 

  1. Adhesive Dispersion – Type Systems:

  • This is a streamlined version of membrane permeation-controlled systems. 

  • In this system, the drug and particular excipients are added directly to the adhesive solution.

  •  The solvent is then removed by drying the thin films that were cast after they were combined and mixed. 

  • The drug reservoir (film) is then sandwiched between the rate-regulating adhesive polymer membrane and the banking laminate. 

  • The rate of drug release from this system is given by, 

    • dq/dt = Cr.Ka/r .Da/ha

  • Where 

    • Ka/r = Partition co-efficient for interfacial partitioning of drug from reservoir layer to adhesive layer. 

    • ha= Thickness of adhesive layer.

    • Da= Diffusion Coefficient of a derive layer.

  • Examples: Isosorbide dinitrate releasing TDDS – 24 hr, Used in Angina Pectoris Verapamil releasing TDDS – 24 hrs, used in Hypertension.

  1. Microreservoir dissolution controlled TDD system:

  • It's a hybrid system of reservoir and matrix dispersion drug delivery. 

  • The drug reservoir is formed in this system by first suspending the drug solids in an aqueous solution of a water-miscible drug solubilizer, such as polyethylene glycol, and then homogeneously dispersing the drug suspension with a controlled aqueous soluble lipophilic polymer using high shear mechanical force to form thousands of un-leachable microscopic drug reservoir. 

Commonly asked questions:

  1. What is TDDS? Discuss various formulation approaches for making a TDDS.

  2. Write a short note on,

    1. Polymer membrane permeation controlled TDDS.

    2. Polymer matrix diffusion controlled TDDS.

    3. Adhesive Dispersion – Type Systems.

    4. Microreservoir dissolution controlled TDDS.
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