**12.3. Worked examples**

**Problem 1:** Describe how you can determine the partition coefficient of a labeled drug

**Solution:** The partition coefficient of a drug is its differential distribution between the hydrophobic and hydrophilic phases. The distribution of the drug between these two phases can be determined by allowing equilibration of a radioactively labeled drug between aqueous buffer containing the drug and a cell membrane preparation obtained by homogenization and fractionation of a tissue sample. The ratio of drug concentration in the membrane to the concentration in the aqueous phase gives the partition coefficient.

**Problem 2:** Describe how you can demonstrate transport across membranes

**Solution:** Erythrocyte 'ghosts' or self sealing micelles formed when the erythrocytes release cytoplasmic contents upon exposure into a hypotonic solution can be used to study the uptake or release of labeled molecules across erythrocytes. When the ghosts are prepared in 14C glucose, it will be possible to monitor the rate of release or uptake of the labeled 14C glucose from the membranes into the aqueous environment. These 'ghosts' can also be used to study the uptake of various molecules at various concentrations under different conditions of temperatures and the presence of inhibitors for specific molecular uptake.

**Problem 3:** A 120mg per kg dose of a drug was injected intravenously and its concentration (mg/L) monitored regularly over time. When log 10 C was plotted vs time (h) a linear response was obtained with a slope of – 0.08 and an extrapolated y-intercept of 1.3. Calculate the following pharmacokinetic parameters;

**i.** elimination rate constant (k)

**ii.** initial concentration of the drug in blood plasma (Co)

**iii.** volume of distribution (Vd)

**iv.** half-life of drug elimination (t ½).

Solution*:*

**12.1. Revision exercise 1**

494 Drug Discovery

**12.2. Revision exercise 2**

**12.3. Worked examples**

[Co].e-kt

**1.** Discuss the absorption of ά-D- Ribose-5-phosphate, given that the two ionizable hydroxyl groups of the monophosphate ester ribose have pKa values of 1.2. and 6.6. The fully

H

H

OH OH OH

H CH2

protonated form of ά-D-ribose 5-phosphate has the following structure;

O

P O

**2.** Using specific examples of drugs, justify their various routes of administration.

If the concentration of a drug in plasma decreases at a rate that is proportional to its initial concentration, give an expression that describes this relationship and hence show that [Ct

**Problem 1:** Describe how you can determine the partition coefficient of a labeled drug

concentration in the aqueous phase gives the partition coefficient.

**Problem 2:** Describe how you can demonstrate transport across membranes

temperatures and the presence of inhibitors for specific molecular uptake.

following pharmacokinetic parameters;

**Solution:** The partition coefficient of a drug is its differential distribution between the hydrophobic and hydrophilic phases. The distribution of the drug between these two phases can be determined by allowing equilibration of a radioactively labeled drug between aqueous buffer containing the drug and a cell membrane preparation obtained by homogenization and fractionation of a tissue sample. The ratio of drug concentration in the membrane to the

**Solution:** Erythrocyte 'ghosts' or self sealing micelles formed when the erythrocytes release cytoplasmic contents upon exposure into a hypotonic solution can be used to study the uptake or release of labeled molecules across erythrocytes. When the ghosts are prepared in 14C glucose, it will be possible to monitor the rate of release or uptake of the labeled 14C glucose from the membranes into the aqueous environment. These 'ghosts' can also be used to study the uptake of various molecules at various concentrations under different conditions of

**Problem 3:** A 120mg per kg dose of a drug was injected intravenously and its concentration (mg/L) monitored regularly over time. When log 10 C was plotted vs time (h) a linear response was obtained with a slope of – 0.08 and an extrapolated y-intercept of 1.3. Calculate the

HO O

OH

**i.** Slope = - k/2.303, therefore, k = -2.303x -0.08 = 0.184

**ii.** Co = Antilog of 1.3 = 10 1.3 = 20 mg/L

**iii.** Volume of distribution (Vd) = Do/Co = 120/20 = 6 L

**iv.** The half-life of elimination t1/2 = 0.693/k = 0.693/0.184 = 3.7 hours.

Once the apparent volume of distribution is known for a particular drug the amount of drug that must be given to achieve a desired concentration can be determined from

Do = Co.Vd

] =

**Problem 4:** You have been given the following data based on a 65 kg patient; t1/2 of drug X = 4.5hrs, Vd = 0.56L/kg, Cmin the = 5mg/L, C tox = 20mg/L and Css = 10mg/L; calculate:-

**i.** Drug clearance from the body

**ii.** Average rate of drug intake (Dosing rate)


**vi.** Loading dose at steady state concentration

#### **Solution**

Since t1/2 = 0.693/ k; k= 0.693/ t1/2 i.e. 0.693/4.5=0.154, and V= 0.56 x 65 = 36.4.

**i.** Total drug clearance = kV = 0.154 x 36.4 = 5.6L/h = 93.3ml/min.

**ii.** Average rate of drug intake = rate elimination constant

= kxVxCss

= 0.154 x 36.4 x 10

= 56 mg/h

**iii.** Maintenance dose

= (Ctox - Cther). V

= (20 – 5) x 36.4 = 546 mg

**iv.** Maintenance interval = maintenance dose/ rate of elimination

= 546/56

= 9.75 hrs

For a practical loading schedule, the maintenance interval should be lowered to say 8.0 hrs and the maintenance dose reduced proportionately: = 546 x 8/9.75 ~ 437 mg.

**Practical problem 2**

Liver damage can be induced by 20% w/v carbon tetrachloride. Given 10mg/ml pentobarbitone and 10mg/ml Phenolbarbitone, design an experiment that demonstrates that the duration of

Introduction to Biochemical Pharmacology and Drug Discovery

http://dx.doi.org/10.5772/52014

497

The development of new drugs over the past 30 years has revolutionalized the practice of medicine and has for instance seen the increased use of new anti-hypertensives and drugs that reduce cholesterol synthesis or dissolve blood clots which led to a 50% reduction in the number

These are the classical approaches to drug discovery that do not initially involve detailed

This is the discovery of drugs based on traditional medical knowledge. The best example is the documented analgesic effects of extracts from opium poppy that led to the isolation of

This is the accidental discovery of novel drugs based on the ingenuity of a scientist investi‐ gating a problem initially unrelated to the observed phenomenon; examples of such discov‐ eries include the observation by Alexander Flemmings that penicilliummould could inhibit

Discovery of therapeutic usefulness of a side effect e.g. clonidine originally used as a nasal decongestant was found to have antihypertensive properties while, the hypoglycemic effects of sulphonamides used in the treatment of typhoid fever led to the development of structurally

An example of discovery arising from studies of endogenous agents in test animals is the anticoagulant action of the venom from the Malayan viper that led to the identification of the

These are those approaches that form a basis for the rational design of drugs and include the

action of short acting barbiturates are dependent on the integrity of the liver.

of deaths from cardio-vascular diseases and stroke among other diseases.

morphine from the plant and the subsequent synthesis of related analgesics.

the growth of bacteria. This finding led to the discovery of antibiotics.

related sulphonylureas as oral hypoglycemic drugs.

**Discovery from effects of endogenous agents in test animals**

**13. Drug discovery and preclinical trials**

**13.1. Conventional approaches to drug discovery**

scientific study they include the following;

**Traditional knowledge approach**

**Discovery through serendipity**

anticoagulant ancrod.

following;

**Bioprospecting**

**Modern approaches to drug discovery**

**v.** The initial loading dose

= Ctox.V

= 36.4 x 20

= 728 mg

**vi.** The loading dose at steady state

= Css x V

= 36.4 x 10 = 364 mg

#### **Practical problem 1**

The analytical method of assaying paracetamol relies on the introduction of a nitro group into the molecule after the removal of plasma proteins through precipitation. The resultant nitrophenol compound which is formed has a deep yellow colour in an alkaline medium and absorbs at 430nm Figure 12.

**Figure 12.** Formation of a chromogenic nitro compound from an analgesic Acetaminophen

**i.** Describe how you would construct the standard curve for determination of parace‐ tamol concentration.

K=ln X1

X t1-t2

**2.** Design an experiment that would enable you to determine the t1/2 of paracetamol
