Upload
dinhkiet
View
223
Download
5
Embed Size (px)
Citation preview
UNIT 3
ELIMINATION RATE FROM URINARY DATA
S.SANGEETHA., M.PHARM., (Ph.d)DEPARTMENT OF PHARMACEUTICSSRM COLLEGE OF PHARMACYSRM UNIVERSITY
Calculation of kel from Urinary Excretion Data after I.V. Injection
kel can be calculated from urinary excretion data.The excretion rate of the drug is assumed to be first order.ke is the renal excretion rate constant.Du is the amount of drug excreted unchanged in the urine.
2
Rate of Drug Excretion in the Urine
Equations
But DB= DB0e‐kelt
Therefore,
5
dDu = keDB
dt
tkBe
u eleDkdt
dD −= o
tkDkdt
dDelBe
u −= olnln
Example
7
Time Du(mg)
Du/t mg/hr t* (hr)
0.25 160 160/0.25 640 0.125
0.5 140 140/0.25 560 0.375
1.0 200 200/0.5 400 0.750
2.0 250 250/1 250 1.50
4.0 188 188/2 94 3.0
6.0 46 46/2 23 5.0
Difference between t and t*t is the time interval for collection of urine
sample.
t* is the midpoint of collection period.
Assuming renal clearance is constant, Du/t
is proportional to plasma drug conc, and
plotting Du/t vs. t* is like plotting Cp vs. time.
The measured urinary excretion rate reflects
the average plasma concentration during
the collection interval.8
Why t* ?Because the drug urinary excretion rate (dDu/dt) cannot be determined experimentally at any given instant.In practice, urine is collected over a specified time interval, and the urine specimen is analyzed for drug.An average urinary excretion rate is then calculated for that collection period.The average dDu/dt is then plotted against the average time (t*).
9
Determination of the non-renal rate constant (knr)
knr= is the elimination rate constant for any route of elimination other than renal excretion.
kel ‐ ke = knrSince drug elimination occurs mainly through renal excretion and metabolism,
knr ≈ kmkel = ke + km
10
Determination of renal clearanceRenal clearance, ClR, is defined as the volume of plasma that is cleared of drug per unit of time through the kidney
11
delT VkCl ×=
deR VkCl ×=
Sigma-Minus MethodAlso called the Amount of Drug Remaining to
be Excreted Method.
It is an alternative method for the calculation
of kel from urinary excretion data.
It is more accurate than the previous method.
ke/kel is the fraction of drug excreted
unchanged in the urine.
(ke/kel)*Dose= total amount of drug excreted
unchanged in the urine.12
Sigma-Minus Method (cont)Equations
Where,
o Du is the cumulative amount of drug excreted
unchanged in the urine until time t.
o (1‐ e‐kelt) is the fraction of drug lost from the
body.13
)1(0 tk
el
eu
elekDkD −−=
Sigma-Minus Method (cont)The amount of drug that is ultimately excreted at time infinity will be equal to Du
∞
Du∞ = ke/kel (D0) (2)
By substituting in the previous equation (1)Du
∞ ‐ Du = Du∞ e‐kelt (3)
To obtain a linear equation:Ln (Du
∞ ‐ Du) = ln Du∞ ‐ kelt (4)
Where, (Du∞ ‐ Du) is the amount of drug remaining
to be excreted. 14
ExampleUse these data to calculate kel
16
Time (hr) Du (mg) Du (cum) Du∞ - Du
0.25 160 160 824
0.5 140 300 684
1.0 200 500 484
2.0 250 750 234
4.0 188 938 46
6.0 46 984 0
Cumulative Amount of Drug Excreted in the UrinePlot
17
Du∞
Cum
ulat
ive
amou
nt e
xcre
ted
Time
One needs to collect urine samples for a minimum of 7-10 half-lives of the drug to assure all the drug is excreted into the urine.
Renal clearance
Renal clearance can be determined from model independent equation
18
∞
∞
=0][AUC
DCl uR
Fraction of drug excreted
The fraction of drug excreted unchanged in the urine (fe) can be calculated as follows:
19
el
eue k
kDoseDf ==
∞
TeTel
eR ClfCl
kkCl ==
Comparison between the Rate and the Sigma-Minus Method1‐ In the rate method, Du
∞ need not be known, and the loss of one urine specimen does not invalidate the entire study.
2‐ The sigma‐minus method needs accurate determination of Du
∞ which requires urine collection until drug excretion is complete.
3‐ Fluctuations in the rate of drug elimination and experimental errors (such as incomplete bladder emptying) cause considerable departure from linearity in the rate method. 20
Comparison (cont)4‐ The sigma‐minus is less affected by fluctuations in the rate of drug elimination.
5‐ The rate method is applicable to zero‐order elimination process, while sigma‐minus method is not.
6‐ The ke can be obtained from the rate method but not from the sigma‐minus method.
21
Problems in Obtaining Valid Urinary Excretion Data1‐ A significant fraction of unchanged drug must be excreted in the urine( at least 20 % ).2‐ The assay technique must be specific.3‐ Frequent sampling is necessary for a good curve description.4‐ Urine samples should be collected until almost all drug is excreted(7 t half)5‐ Variation in urinary pH and volume cause significant variation in urinary excretion rates.6‐ Subjects should be instructed to the importance of complete bladder emptying.
22