Instant Release (IR) Products are designed to provide relief to the patient at the earliest. These products make the Drug available in the body fluids at a relatively faster speed and hence the Drug is available immediately at the desired location for the action.
However, though the IR and Instant relief (Ir) looks linear and the release of Drug from the formulation ( with respect to Speed & Quantity both) is assumed to be directly proportional to the amount of Drug at the site of action. So, one can expect the arrival of the entire Drug quantity at the site of action "Faster" than the normal formulation of the same Drug.
Say , if X quantity of Drug is released in Y time, then X will be available at the site of action in Y1 min. Whereas, if formulation is not IR, the X quantity of Drug Will reach the site of action in Y2 min ( Y2 >Y1). Please note that here it is assumed that the Quantity of Drug that reaches the site of action remains the same.
This, however may not be true in all the cases and must be studied during the Drug product design to take care of the extra interactions with the surrounding environment.
Now, let's look at the other side of the story. When Drug is released "Instantly" or faster, more quantity of Drug will be available in solution form in the body fluid. The chances of interaction with fluid and cellular environments will be many fold higher than so called "Normal" release formulation. The chemical interactions are at peak when substance is present in solution form. This immediate "load" of soluble form of Drug post instant release would make the Drug interactions with surrounding fluids & environment multifold higher. These interactions may include degradation, binding etc. This enhanced chemical interactions may lead to variations in the amount of Drug that is expected at the site of action. Therefore, IR formulations may need an additional mechanism to balance this variation and get the effect as expected from the "Normal" formulation of the IR Products.
In the IR formulations, one cannot expect the same amount of Drug at the site of action considering several times increase in the Drug interactions with surrounding environment.
Hence the IR and Ir correlation may require a vigilant approach during design and development to take care of such extra interactions.
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