3.) Pharmaceutical Formulations 2

3.) Pharmaceutical Formulations 2

1. INTRINSIC DISSOLUTION RATE (IDR)

   • What is Intrinsic Dissolution Rate?
     • The rate of dissolution is dependant on many variables such as pH, viscosity, temperature etc. Intrinsic dissolution rate is independant of these variables
   • How is it calculated?
     • The Noyes Whitney equation is rearranged so that area variable is kept constant within the equation
     • The equation can then be further simplified by introducing the intrinsic dissolution rate constant (K)
       • K takes care of D/h, Units = cm/s
     • Simplied equation therefore:
   • Example Questions
     • 1.) When calculating anything using the IDR equation, you have to first decide whether or not sink conditions apply. This is done by working out 10% of Cs and then seeing how it compares with C. If C less than 10% then sink applies
     • 2.) If sink conditions apply then Cs-C becomes simply Cs
     • 3.) Then convert all of the units so that they are the same or similar
     • 4.) Plug the values into the equation. The equation may need to be rearranged
   • Why do you measure IDR?
     • This is measured to know what the rate of dissolution would be if all variables were kept at a certain constant. So that if the rate of dissolution does change we know that at least one of the variables has changed
       • This allows you to know how new drugs will act when all variables are kept constant
   • Dissolution Apparatus
     • What does it do?
       • This allows you to see how drugs will act when in conditions similar to the human body more specifically the small intestines
     • Methods
       • Static Disc Method
         • Can also be rotating disc method
         • Apparatus
           • Temperature is 37 degree as this is human body temperature
           • Rotating paddle is used to make the fluidity similar to that of the intestines
           • Drug is compressed to ensure there are no pores in the drug so solution cannot affect dissolution

2. Hixson-Cromwell Cube Root Law

   • Why is this used to the Noyes Whitney Equation?
     • The Noyes Whitney equation assumes that the surface area of the particles remain constant but in practice they dont. This equation takes into account the changing surface area
   • What is the equation?
     • It can be arranged in such an equation for a straight line graph
     • The equation says that the cube root of the undissolved mass at time, t is the same as the cube root of mass at the start minus cube root dissolution constant
   • Example Question
     • For this question you fill out a table like so. first you work out the undissolved using the concentration. Then find the dissolved. Finally cube root the dissolved to to get the final answer

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