Bioavailability of a Drug

Bioavailability is the drug's availability to the patient after administration in the blood, which will eventually cause an action. When a drug is administered, it undergoes a process of absorption, which is crucial for its bioavailability. Doctors should properly understand a drug's bioavailability as they have to prescribe medicine in the correct doses to their patients.

The process of bioavailability is not immediate but rather gradual, leading to an increase in plasma concentration until it reaches a maximum level and then starts to decline. This understanding of bioavailability is fundamental to comprehending the drug's action in the body. 

Plotting a graph to show the plasma drug concentration is known as a "Bioavailability Graph."

• The Y-axis shows the plasma drug concentration, and the X-axis shows the time in hours.
• When a drug is administered, the plasma drug concentration peaks gradually and starts falling, as shown in the graph, which has a bell-shaped curve.
• The concentration at which we see the maximum drug concentration is the Cmax. The time it takes to reach the Cmax is called the Tmax.
• The minimum effective concentration (MEC) is the plasma drug concentration level at which the drug becomes effective. This is called the "onset of action" of the drug.
  • When the drug level falls below the MEC, it stops working. 
• The point between the onset of action and when the drug stops working is called the “duration of action.”
• The minimum effective concentration (MEC) for adverse effects (AE) is the minimum drug concentration at which adverse effects start occurring. The range between the MEC and the MEC for AE is called the therapeutic window.
  • The drug should be given within this therapeutic window to remain effective without A/E.
• An example will make this concept clearer. For example, the dosage of paracetamol in an adult is 500mg. If an adult takes only 100 mg, then it will not achieve the MEC and thus not work. If a higher dose of 2–3 gm of paracetamol is given, then it exceeds the MEC for AE and thus can lead to AE. Thus, the drug should be given within the therapeutic window.
• The area under the bell curve is called the area under the curve (AEC).
• Tmax mainly gives the rate of drug absorption.
• AUC gives the extent of drug absorption.

Also Read: Drug Label And FDC

Formula to Calculate the Bioavailability

• When the drug is given via the IV route, its plasma level falls below the maximum level. There's no bell curve because there's no absorption. This is the AUC of the IV route. The Tmax is zero, as the drug works immediately.
• When the drug is given via the oral route, the drug concentrations increase, peak, and fall. This is the AUC for the oral route.
• Bioavailability (BA) is calculated as follows:

Also Read: Anti-Diabetic Drugs: Types and Mechanism

Increasing AUC (Extent of Drug Absorption)

• When a drug is given orally, the graph goes up, reaches the Cmax, and then comes down. The bell curve shows the AUC.
• If the absorption is higher, the drug concentration will be higher. The graph decreases after reaching Cmax due to drug elimination via metabolism and excretion.
• To increase the AUC, either the absorption of the drug needs to be increased, the first-pass metabolism should be decreased, or the elimination of the drug should be decreased.

Also Read: Pharmacokinetics: Absorption, Distribution, Metabolism, Excretion

Bioequivalence

• Let us understand this using an example of a company that sells paracetamol under the brand name Dolo-650. If another company wants to develop the brand "P-650" for paracetamol, it should have a similar bioavailability as Dolo-650 to receive approval.
• The new drug should have an almost similar bioavailability of +/-20% as that of the comparator drug, i.e., the rate and extent of absorption of the drug should be similar. This is important to have a similar effect and avoid AE.

Also Read: Anti Cancer Drugs: Mode of action and Adverse effects

Summary

This is a summary of the information given to you for your last-minute revision of the topic for all your competitive exams, such as NEET-PG, NexT, FMGE, and INICET.

• The y-axis in a bioavailability graph shows the drug's plasma concentration, and the x-axis gives the time.
• The rate of absorption is given by Tmax.
• AUC gives the extent of absorption.
• To increase the AUC, we need to increase the absorption, decrease the first pass, decrease the metabolism, or decrease the drug's excretion.
• Bioequivalence: The new drug should have a similar bioavailability as the original drug with a variation of +/- 20% to get approved.
• The onset of the drug's action is when it reaches the MEC. The drug stops being effective after it falls below the MEC. The duration between the MEC and drug effect stoppage is the duration of action.
• The IV route has an immediate onset of action since the Cmax can be achieved immediately.

Also Read: 

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