Pharmacology Equations

Question 1

M&M Kinetics

Answer 1

Km - the conc of substrate needed to achieve 50% Vmax
(higher Km means lower affinity)

Vmax - maximum velocity achieved (represents efficacy)

Question 2

How do competitive v. non-competitive inhibitors change M&M plot?

Answer 2

Comp - same Vmax but higher Km (need more substrate to get to same effect); less potent

Non-comp - lower Vmax but Km is unchanged (less efficacy - cannot overcome by inc substrate)

Question 3

Lineweaver-Burke Plot

Answer 3

1/V by 1/ conc substrate

X intercept - 1/-Km
Y intercept - 1/ Vmax
Slope - Km / Vmax

Question 4

How do competitive v. non-competitive inhibitors change LB plot?

Answer 4

Comp - same Vmax so same y-intercept but x-intercept is closer to 0 representing an inc Km

Non - comp - lower Vmax so higher y- intercept but same Km so same x-intercept

Question 5

Vd

Answer 5

Vol of Distribution

Vd = (amount of drug in body) / plasma drug conc

Question 6

CL

Answer 6

Clearance

Vol cleared per unit time

CL = (rate of elimination) / plasma drug concentration

CL = Vd x Ke (elimination constant)

Question 7

Half - Life

Answer 7

t 1/2 = (.693 x Vd) / CL

IN FIRST ORDER ELIMINATION (most drugs)

Question 8

Loading Dose

Answer 8

Cp x Vd / F

Cp is desired conc in plasma

F is bioavailability (1 if IV)

Question 9

Maintenance Dose

Answer 9

Cp x CL x T / F

Cp is desired conc in plasma

T is dose interval

**Maintenance dose dec if renal or liver failure b/c dec CL

Question 10

What does time to steady state depend on?

Answer 10

HALF LIFE not dose or dose frequency

Question 11

Therapeutic Index

Answer 11

TD50 / ED50

median toxic dose / media effective dose

Question 12

Which 4 drugs have low therapeutic index?

Answer 12

Warfarin
Theophylline
Digoxin
Lithium

“Warning, these drugs lethal”

Question 13

Zero Order Elimination (3 known drugs)

Answer 13

Same amount eliminated per unit time (independent of conc)

PEA - phenytoin, ethanol, aspirin

Question 14

First Order Elimination

Answer 14

Flow-dep so rate of elimination dir proportional to conc of drug; not same amount but same fraction eliminated per unit time

Question 15

General Principles of Anesthetics

Answer 15

Less water soluble means rapid induction and recovery time

Less lipid soluble means less potent

Potency = 1/ MAC

Minimal alveolar concentration - conc of inhaled anesthetic needed to prevent 50% subjects from moving in response to noxious stimulus

Question 16

How many half-lives to reach steady state?

Answer 16

1 half life = 50% steady state

2 half lives = 75%

3 half lives = 87.5%

4-5 half lives = steady state