Pharmacokinetics and mechanisms of tracer uptake

Question 1

Define Pharmacokinetics

Answer 1

The process by which a drug is absorbed, distributed, metabolized, and eliminated by the body.

Question 2

Describe initial bio-distribution

Answer 2

• Simple dilution in a compartment, such as plasma,
• Diffusion process along concentration gradient
• Substance crosses permeability barrier.

Question 3

Effects of bio-distribution

Answer 3

Tissue distribution in the body not necessarily the distribution of the intact radio-pharmaceutical - may result from metabolism of the starting material

Question 4

What is passive diffusion?

Answer 4

Moving from higher to lower concentration – down a concentration gradient.

Does not require energy

Question 5

What is a Facilitated diffusion?

Answer 5

Proteins in the cell membrane are used to move molecules - grab a molecule passing then shift their position to bring them into the cell.

Still moving into the cell, passive trans through lipid membrane ex. HMPAO in brain

Question 6

What is active transport?

Answer 6

Proteins embedded in the cell’s lipid bi-layer move specific molecules and ions in and out of the cell.

Often have to work against a concentration gradient by pumping ions from low to higher concentration.

ex. pertechnetate uptake in thyroid, hepatobiliary uptake

Question 7

Describe diffusion

Answer 7

• From a higher to a lower concentration
• Rate ∝ concentration of the radiotracer.
• Net movement of molecules continue until concentration is in equilibrium.

Xe admin through inhalation

Question 8

Describe capillary blockade

Answer 8

Tracer cannot pass through smallest capillaries, IV solution blocks 0.3% available pulmonary vessels. Filtered out in pulmonary circulation

Microspheres and macroaggregates
- Metabolic degradation and phagocytosis

Question 9

Define phagocytosis

Answer 9

Particles of colloid dimensions are engulfed by phagocytic cells

Particulate comes in contact with phagocytic cell: are recognised, attach to the cell surface, and are engulfed.

Kupffer cells of liver, macrophages of spleen and bone marrow up to 10um in size

Question 10

Describe ion exhange / chemisorption

Answer 10

Phosphonates accumulate in hydroxyapatite crystal matrix.

Rapid bone uptake proportional to bone turnover:

• Increased osteoblastic activity
• Increased regional blood flow

Question 11

Describe facilitated diffusion

Answer 11

All cells use glucose to generate metabolic energy.

Tumour cells have increased rates of anaerobic and aerobic glycolysis compared to normal tissue.

18F FDG is metabolically trapped

Question 12

Describe Metabolic incorporation

Answer 12

Metabolic pathway same as for natural non-radioactive element

Ex 131I and 123I

Question 13

Examples of active transport

Answer 13

201 Thallium potassium analogue:
Thallous ions taken up by active transport system for potassium

Hepatobiliary Studies:
99mTc. IDA transported by the anion transport system of the liver.

Question 14

Describe Autologous products

Answer 14

Cells isolated from the body, labelled,
then injected back.

Example RBC labelled in vitro or in vivo 99mTc - eventually filtered by spleen
WBC (infection)
Platelet (thrombus site)

Question 15

Describe receptor binding agents

Answer 15

binding analogue of another molecule

ex 123I Ioflupane (FP-CIT) DaTSCAN: Analogue of cocaine

Binds to dopamine active transporter (DaT) sites on presynaptic terminals of the dopaminergic neurones in the corpus striatum.

Question 16

Describe antibodies

Answer 16

Proteins in bloodstream recognise and attach to proteins ‘antigens’ on surface of viruses, bacteria, tumour cells

Allows other components of immune system to destroy
and remove the foreign invaders.

A low amount are taken up by tumors