9.3
Desenho do estudo de biodisponibilidade: biodisponibilidade absoluta versus relativa
A biodisponibilidade é um parâmetro farmacocinético essencial que quantifica a proporção de um fármaco administrado que alcança a circulação sistêmica…
Os dados de biodisponibilidade medidos pela área sob o gráfico de concentração plasmática versus tempo do medicamento são obrigatórios para submissões regulatórias.
Esses estudos fornecem biodisponibilidade absoluta e relativa.
A biodisponibilidade absoluta mede a proporção de fármaco ativo na circulação sistêmica após uma administração extravascular em comparação com a via intravenosa, o que representa 100% de biodisponibilidade.
É a razão entre a AUC do medicamento extravascular corrigida pela dose e a AUC do medicamento intravenoso. Para medicamentos orais, Fabs é calculado da seguinte forma. Aqui, uma faixa de Fabs de 0 a 1 representa uma taxa de absorção sistêmica de 0 a 100%.
A biodisponibilidade relativa avalia a exposição sistêmica de um medicamento em uma formulação de teste em relação a uma formulação de referência. Frel compara a área sob as curvas de ambas as formulações, conforme visto na equação.
Esses estudos garantem que novas formulações tenham biodisponibilidade sistêmica consistente de medicamentos em comparação com formulações aprovadas já em uso.
Além disso, eles caracterizam os efeitos dos alimentos e as interações medicamentosas, determinando a biodisponibilidade do medicamento oral com e sem alimentos ou o medicamento que interage.
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Q1: What is the difference between absolute and relative bioavailability?
Absolute bioavailability (Fabs) measures the proportion of active drug reaching systemic circulation after extravascular administration, such as oral dosing, compared to intravenous administration, which represents 100% bioavailability. Relative bioavailability (Frel) compares systemic drug exposure between a test formulation and a reference formulation. Both are calculated as dose-normalized ratios of their respective area under the curve values.
Q2: How is absolute bioavailability calculated for oral drugs?
Absolute bioavailability for oral drugs is computed as the dose-corrected ratio of extravascular drug area under the curve to intravenous drug area under the curve. The resulting Fabs value ranges from 0 to 1, representing systemic absorption rates from 0% to 100%. This calculation accounts for differences in administered doses between routes to provide an accurate comparison of drug absorption efficiency.
Q3: Why are relative bioavailability studies important for new drug formulations?
Relative bioavailability studies ensure new formulations maintain consistent systemic drug bioavailability compared to approved reference formulations already in clinical use. These studies are essential for assessing generic drugs and alternative delivery systems, confirming therapeutic equivalence between formulations. They help maintain clinical efficacy and safety profiles while supporting regulatory submissions and approval processes.
Q4: What role does the area under the curve play in bioavailability assessment?
The area under the drug plasma concentration-versus-time curve (AUC) is the mandatory measurement for regulatory bioavailability submissions. AUC quantifies total systemic drug exposure and serves as the basis for calculating both absolute and relative bioavailability values. Comparing dose-normalized AUC values between different administration routes or formulations determines the proportion of active drug reaching systemic circulation.
Q5: How do bioavailability studies assess food and drug interactions?
Bioavailability studies characterize food effects and drug-drug interactions by measuring oral drug bioavailability under different conditions: with and without food, or with and without interacting drugs. These evaluations determine how concurrent substances influence systemic drug availability. Results guide dosing recommendations and co-administration practices, optimizing therapeutic outcomes and minimizing adverse effects in clinical settings.
Q6: What does an Fabs value of 0.5 indicate about drug absorption?
An Fabs value of 0.5 indicates that 50% of the administered oral dose reaches systemic circulation compared to intravenous administration. Since Fabs ranges from 0 to 1, representing 0% to 100% systemic absorption, a value of 0.5 reflects moderate bioavailability through the oral route. This suggests that first-pass metabolism, permeability limitations, or other physicochemical factors reduce the proportion of active drug entering systemic circulation.
Q7: Why is intravenous administration considered the reference standard for bioavailability studies?
Intravenous administration is the reference standard because it delivers 100% of the drug directly into systemic circulation, bypassing absorption barriers and first-pass metabolism. This provides a baseline for comparing systemic exposure achieved through other routes. All extravascular bioavailability calculations are normalized against intravenous AUC values, making it the gold standard for assessing drug delivery efficiency through alternative administration routes.