Drug A is to be administered at constant rate of 25 mg/hr. The following PK parameters are estimated for this pt: CLs = 15 mL/min, V = 31.5 L, K = 0.029hr. a. Calculate this patient’s plasma concentration at 6 hr and 12 hr after the infusion has begun. b. If the infusion is continued to steady state, what would Css be c. what would be the plasma concentration 24 hr after the infusion is stopped?

a. To calculate the plasma concentration at 6 hours and 12 hours after the infusion has begun, we need to use the formula for the concentration of a drug in the body during a constant rate infusion before reaching steady state:

C = (Rate of infusion / Clearance) * (1 - e^(-K*t))

Where:

• Rate of infusion = 25 mg/hr
• Clearance (CL) = 15 mL/min = 0.015 L/hr
• K = 0.029 hr^-1
• t = time in hours

For t = 6 hours: C6 = (25 mg/hr / 0.015 L/hr) * (1 - e^(-0.029*6)) = 1667 mg/L * (1 - e^-0.174) = 1667 mg/L * (1 - 0.840) = 266.72 mg/L

For t = 12 hours: C12 = (25 mg/hr / 0.015 L/hr) * (1 - e^(-0.029*12)) = 1667 mg/L * (1 - e^-0.348) = 1667 mg/L * (1 - 0.706) = 490.62 mg/L

b. The steady state concentration (Css) is achieved when the rate of drug input equals the rate of drug elimination. It can be calculated using the formula:

Css = Rate of infusion / Clearance

Css = 25 mg/hr / 0.015 L/hr = 1667 mg/L

c. The plasma concentration 24 hours after the infusion is stopped can be calculated using the formula for the concentration of a drug in the body after stopping a constant rate infusion:

C = Css * e^(-K*t)

Where:

• Css = 1667 mg/L
• K = 0.029 hr^-1
• t = time in hours after stopping the infusion

For t = 24 hours: C24 = 1667 mg/L * e^(-0.029*24) = 1667 mg/L * e^-0.696 = 1667 mg/L * 0.499 = 833.33 mg/L