Tissue distribution half-life is expressed as:

Tissue distribution half-life is expressed as:

• A. half-life = 0.693/kT
• B. half-life = kT/0.693
• C. half-life = (Q/Vt)/0.693
• D. half-life = 0.693 Kp/(Q/Vt)

Answer: (A)

In pharmacokinetics, a first-order process has the half-life t1/2 = 0.693/k, where k is the rate constant of that process. For tissue distribution, the rate constant that governs movement into tissues is kT, so the tissue distribution half-life is t1/2, distribution = 0.693/kT. This expression gives the correct units (time) and directly ties how fast distribution happens to how long it takes for the amount in tissue to halve. The other forms mix in parameters like Q (blood flow), Vt (tissue volume), or Kp (partition coefficient) in ways that don’t produce a time unit for half-life, or invert the relationship (which would yield a rate, not a time). Therefore, 0.693 divided by kT is the appropriate expression.

In pharmacokinetics, a first-order process has the half-life t1/2 = 0.693/k, where k is the rate constant of that process. For tissue distribution, the rate constant that governs movement into tissues is kT, so the tissue distribution half-life is t1/2, distribution = 0.693/kT. This expression gives the correct units (time) and directly ties how fast distribution happens to how long it takes for the amount in tissue to halve.

The other forms mix in parameters like Q (blood flow), Vt (tissue volume), or Kp (partition coefficient) in ways that don’t produce a time unit for half-life, or invert the relationship (which would yield a rate, not a time). Therefore, 0.693 divided by kT is the appropriate expression.