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DERIVE THE EQUATION TO
DETERMINE THE PLASMA DRUG
CONCENTRATION AT STEADY
STATE LEVEL AFTER MULTIPLE
INJECTION.
PREPARED BY:
MD. AMZAD HOSSAIN
𝐶 𝑚𝑎𝑥
α
𝐶 𝑎𝑣
α
Plasma level 𝐶 𝑚𝑖𝑛
α
1 8 12 16
Time
Concentration of drug in the body after 1st dose , 2nd dose ……. nth dose can be determined by the following ways:
Drug concentration immediately after 1st dose—
𝐶 𝑜. 1=
𝐷𝑜
𝑉
At the end of the 1st dose 𝐶τ. 1 =
𝐷𝑜
𝑉
𝑒−𝑘τ
where , τ = dosing interval, 𝐷𝑜= dose.
let, 𝑒 𝑘τ
= R
so, 𝐶τ. 1 =
𝐷𝑜
𝑉
R --------------- ( 1 )
Immediately after 2nd IV dose, the plasma concentration is ---
𝐶 𝑜. 2 =
𝐷𝑜
𝑉
𝑅 +
𝐷𝑜
𝑉
And at the end of the 2nd dose ----
𝐶 𝑇. 2 = ( 𝐷𝑜
𝑉
𝑅 +
𝐷𝑜
𝑉
) R
=
𝐷𝑜
𝑉
𝑅2
+
𝐷𝑜
𝑉
𝑅
Immediately, after the 3rd dose the plasma concentration is---
𝐶 𝑜. 3 =
𝐷𝑜
𝑉
𝑅2
+
𝐷𝑜
𝑉
R +
𝐷𝑜
𝑉
And at the end of the 3rd dose---
𝐶 𝑜. 3 = (
𝐷𝑜
𝑉
𝑅2
+
𝐷𝑜
𝑉
R +
𝐷𝑜
𝑉
) R
=
𝐷𝑜
𝑉
𝑅3
+
𝐷𝑜
𝑉
𝑅2
+
𝐷𝑜
𝑉
R ------------- ( 2 )
From the above equation (1), (2) and (3) we can see that there is a require among the equation indicating drug concentrations after 1st,
2nd and 3rd dose at dosing interval τ.
So, immediately after 𝑛 𝑡ℎ
dose drug concentration is determined by the following equation :
𝐶 𝑜. n =
𝐷𝑜
𝑉
𝑅 𝑛−1
+
𝐷𝑜
𝑉
𝑅 𝑛−2
+
𝐷𝑜
𝑉
𝑅 𝑛−3
+ ------------ +
𝐷𝑜
𝑉
…………………(4)
And at the end of the 𝑛 𝑡ℎ
dose:
𝐶τ. n =
𝐷𝑜
𝑉
𝑅 𝑛
+
𝐷𝑜
𝑉
𝑅 𝑛−1
+
𝐷𝑜
𝑉
𝑅 𝑛−2
+ ------------ +
𝐷𝑜
𝑉
R
=
𝐷𝑜
𝑉
[𝑅 𝑛
+ 𝑅 𝑛−1
+ 𝑅 𝑛−2
+ ------------ + R] ……………………… (5)
After simplifying the equation (4) and (5) we get…
𝐶 𝑜. n =
𝐷𝑜
𝑉
(
1−𝑅 𝑛
1−𝑅
) ……………… (6)
and, 𝐶τ. n =
𝐷𝑜
𝑉
(
1−𝑅 𝑛
1−𝑅
) R ………….. (7)
=
𝐷𝑜
𝑉
(
1−𝑒−𝑛𝑘τ
1−𝑒−𝑘τ ) 𝑒−𝑘τ
……… (8)
From equation (6), since the plasma concentration at any time “t” during the dosing interval is equal to the plasma concentration at the
start of the 𝑛 𝑡ℎ
dose multiplied by 𝑒−𝑘𝑡
‫؞‬ 𝐶𝑡. n =
𝐷𝑜
𝑉
(
1−𝑒−𝑛𝑘τ
1−𝑒−𝑘τ ) 𝑒−𝑘𝑡
‫؞‬ 𝐶 𝑃 =
𝐷𝑜
𝑉
(
1−𝑒−𝑛𝑘τ
1−𝑒−𝑘τ ) 𝑒−𝑘𝑡
………… (9)
From this equation, the plasma drug concentration at any time “t” can be calculated after the administration of 𝑛 𝑡ℎ
dose.
At steady state, n=α , so, 𝑒−𝑛𝑘τ
≡ 0
𝐶𝑠𝑠 or 𝐶 𝑃
α
=
𝐷𝑜
𝑉
(
1
1−𝑒−𝑘τ ) 𝑒−𝑘𝑡
………… (10)
Where 𝐶 𝑃
α
is the steady state drug concentration at time “t” after the dose.
THE END

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Bio-pharmaceutics and Pharmacokinetics

  • 1. DERIVE THE EQUATION TO DETERMINE THE PLASMA DRUG CONCENTRATION AT STEADY STATE LEVEL AFTER MULTIPLE INJECTION. PREPARED BY: MD. AMZAD HOSSAIN
  • 2. 𝐶 𝑚𝑎𝑥 α 𝐶 𝑎𝑣 α Plasma level 𝐶 𝑚𝑖𝑛 α 1 8 12 16 Time Concentration of drug in the body after 1st dose , 2nd dose ……. nth dose can be determined by the following ways: Drug concentration immediately after 1st dose— 𝐶 𝑜. 1= 𝐷𝑜 𝑉
  • 3. At the end of the 1st dose 𝐶τ. 1 = 𝐷𝑜 𝑉 𝑒−𝑘τ where , τ = dosing interval, 𝐷𝑜= dose. let, 𝑒 𝑘τ = R so, 𝐶τ. 1 = 𝐷𝑜 𝑉 R --------------- ( 1 ) Immediately after 2nd IV dose, the plasma concentration is --- 𝐶 𝑜. 2 = 𝐷𝑜 𝑉 𝑅 + 𝐷𝑜 𝑉 And at the end of the 2nd dose ---- 𝐶 𝑇. 2 = ( 𝐷𝑜 𝑉 𝑅 + 𝐷𝑜 𝑉 ) R = 𝐷𝑜 𝑉 𝑅2 + 𝐷𝑜 𝑉 𝑅 Immediately, after the 3rd dose the plasma concentration is--- 𝐶 𝑜. 3 = 𝐷𝑜 𝑉 𝑅2 + 𝐷𝑜 𝑉 R + 𝐷𝑜 𝑉 And at the end of the 3rd dose--- 𝐶 𝑜. 3 = ( 𝐷𝑜 𝑉 𝑅2 + 𝐷𝑜 𝑉 R + 𝐷𝑜 𝑉 ) R = 𝐷𝑜 𝑉 𝑅3 + 𝐷𝑜 𝑉 𝑅2 + 𝐷𝑜 𝑉 R ------------- ( 2 )
  • 4. From the above equation (1), (2) and (3) we can see that there is a require among the equation indicating drug concentrations after 1st, 2nd and 3rd dose at dosing interval τ. So, immediately after 𝑛 𝑡ℎ dose drug concentration is determined by the following equation : 𝐶 𝑜. n = 𝐷𝑜 𝑉 𝑅 𝑛−1 + 𝐷𝑜 𝑉 𝑅 𝑛−2 + 𝐷𝑜 𝑉 𝑅 𝑛−3 + ------------ + 𝐷𝑜 𝑉 …………………(4) And at the end of the 𝑛 𝑡ℎ dose: 𝐶τ. n = 𝐷𝑜 𝑉 𝑅 𝑛 + 𝐷𝑜 𝑉 𝑅 𝑛−1 + 𝐷𝑜 𝑉 𝑅 𝑛−2 + ------------ + 𝐷𝑜 𝑉 R = 𝐷𝑜 𝑉 [𝑅 𝑛 + 𝑅 𝑛−1 + 𝑅 𝑛−2 + ------------ + R] ……………………… (5) After simplifying the equation (4) and (5) we get… 𝐶 𝑜. n = 𝐷𝑜 𝑉 ( 1−𝑅 𝑛 1−𝑅 ) ……………… (6) and, 𝐶τ. n = 𝐷𝑜 𝑉 ( 1−𝑅 𝑛 1−𝑅 ) R ………….. (7) = 𝐷𝑜 𝑉 ( 1−𝑒−𝑛𝑘τ 1−𝑒−𝑘τ ) 𝑒−𝑘τ ……… (8)
  • 5. From equation (6), since the plasma concentration at any time “t” during the dosing interval is equal to the plasma concentration at the start of the 𝑛 𝑡ℎ dose multiplied by 𝑒−𝑘𝑡 ‫؞‬ 𝐶𝑡. n = 𝐷𝑜 𝑉 ( 1−𝑒−𝑛𝑘τ 1−𝑒−𝑘τ ) 𝑒−𝑘𝑡 ‫؞‬ 𝐶 𝑃 = 𝐷𝑜 𝑉 ( 1−𝑒−𝑛𝑘τ 1−𝑒−𝑘τ ) 𝑒−𝑘𝑡 ………… (9) From this equation, the plasma drug concentration at any time “t” can be calculated after the administration of 𝑛 𝑡ℎ dose. At steady state, n=α , so, 𝑒−𝑛𝑘τ ≡ 0 𝐶𝑠𝑠 or 𝐶 𝑃 α = 𝐷𝑜 𝑉 ( 1 1−𝑒−𝑘τ ) 𝑒−𝑘𝑡 ………… (10) Where 𝐶 𝑃 α is the steady state drug concentration at time “t” after the dose. THE END