I want to prepare a topical and a transdermal patch system of a certain drug which is only available orally at the moment.
How can I calculate the dose of the drug for the patch system? For example, if the oral dose is 10 mg, oral bioavailability is ~90% and volume of Distribution is 200 L, is it possible to calculate the topical dose of the drug in mg/cm2 using any equation/formula?
To calculate the dose, you would first calculate how much of the oral dose reaches the circulation. In your case, 10 mg time 90% bioavailability or 9 mg reaches the blood per tablet. If the tablet is given 3 times a day multiply by this number for the daily dose-- 9 mg x 3/day = 27 mg/day.
Now you need to know what the transdermal bioavailability is. This is typically better than oral, but to make the example more interesting, let's assume there is 20% degradation via transdermal so bioavailability(TD) = 80%. The daily transdermal dose would be 27mg/day /80% = 33.75 mg/day. The release rate should therefore be (33.75mg/day)/(24 hrs/day)= 1.4 mg/hr. As Bahjat discusses, this is the release rate that your patch needs to deliver--as estimated via skin permeation studies using a Franz diffusion cells. You need to achieve this rate at a reasonable patch size--often employing permeation enhancers. Formulate your patch to achieve this to make a viable product. Best of luck!
I don't know the answer. If no one else responds, I would start by looking for all the drugs that are available using two or three of the delivery methods. Then try to calculate your own formula. There are chemicals that you could use that enhance absorption for the topical delivery. DSMA I think is one.
usually for TDDS dose is almost similiar but pattern of release is different in controlled manner. as i know there is no differnt dose calcultion
I think there is no difference in the dose calculation however in the transdermal case you need to know how much drug you need and the area of patch, e.g. if you need 24 mg /day means 1 mg/hr and this will be divided by the area of patch. this is related the formula.
Related the release of drug you need to use Franz cell diffusion system to see the drug release profile.
To calculate the dose, you would first calculate how much of the oral dose reaches the circulation. In your case, 10 mg time 90% bioavailability or 9 mg reaches the blood per tablet. If the tablet is given 3 times a day multiply by this number for the daily dose-- 9 mg x 3/day = 27 mg/day.
Now you need to know what the transdermal bioavailability is. This is typically better than oral, but to make the example more interesting, let's assume there is 20% degradation via transdermal so bioavailability(TD) = 80%. The daily transdermal dose would be 27mg/day /80% = 33.75 mg/day. The release rate should therefore be (33.75mg/day)/(24 hrs/day)= 1.4 mg/hr. As Bahjat discusses, this is the release rate that your patch needs to deliver--as estimated via skin permeation studies using a Franz diffusion cells. You need to achieve this rate at a reasonable patch size--often employing permeation enhancers. Formulate your patch to achieve this to make a viable product. Best of luck!
Keith Jensen and others who responded to my question, thank you very much for your input regarding this issue. I guess the real task is to optimize the release rate rather than to estimate the amount of drug per unit surface area of the patch.
Thanks again. I will attempt it.
There's a great paper just published in Regulatory Toxicology and Pharmacology by Keurentjes and Maibach for the factors associated with influx and permeability of various marketed drugs at Article Percutaneous penetration of drugs applied in transdermal del...
Best wishes, Richard- Badges
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