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PMY2006 > Radiopharmacy > Flashcards

Radiopharmacy Flashcards

1
Q

What are radiopharmaceuticals?

A

Molecules that incoperate 2 components:
1. A radio isotope/nuclide - selected based on properties
2. A non-radioactive component - it may be a simple molecule or ion, with little biological effect, it influences tissue distribution with minimal risk of toxicity

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2
Q

What are radiopharmaceuticals used for in medicine?

A
  1. Diagnostic use - imaging agents (radiolabel which you can detect oputside the body using gamma camera)
  2. Therapeutic use - delivery of therapeutic doses to specific disease sites (much more active as we are delivering high levels of ionising radiation to specific disease sites with the purpose of being able to kill tissue e.g. cancer tumor)
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3
Q

5 Important factors to consider when choosing the ideal radiopharmaceutical

A
  1. Minimum half-life
  2. Mode of decay
  3. Cost and availabilty
  4. Physical properties
  5. Organ/Tissue specificity
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4
Q

What does the half life (t1/2) of a radioisotope mean?

A

The half life of the radioisotope refers to the length of time it takes for half of the radioactivity to decay.

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5
Q

The half-life should be long enough to:

A

o Carry out the chemistry necessary to synthesise the radiopharmaceutical
o Deliver to the clinical site
o Accumulate in the target tissue while clearing non-target organs

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6
Q

However, the half-life should be short enough to:

A

Minimise the radiation dose to the patient (for imaging agents)

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7
Q

Radioisotopes mode of decay: Beta emitters - two types of this decay what are they?

A

Beta minus decay: Isotopes that decay by conversion of a neutron to a proton and a B- particle (electron). (This B- ion is strongly ionising this will be damaging to cells and therefore these radioisotopes are typically used for therapeutic applications.)
Beta plus decay: Isotopes that decay by converting a proton to a neutron and a B+ particle (positron)

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8
Q

Radioisotope mode of decay: Gamma emmiters

A

Isotopes change energy status from a higher (99mTc) to lower (99Tc) energy state, by emitting a gamma ray photon.
The numbers of protons and neutrons are unchanged, so the chemical element remains the same.
γ emission can be secondary to β emission.

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9
Q

Cost and availabilty of radioisotope generators:

A

Radioisotope generators are considered ideal
- Inexpensive and on-site
- Parent isotope decays to a shorter-lived daughter radioisotope which is separated from the parent by ion exchange chromatography or solvent extraction

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10
Q

Cost and availabilty: Particle accelerator or cyclotron

A
  • Most expensive method; one isotope produced at a time
  • Off-site
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11
Q

Cost and availability: Nuclear reactor

A

This method is not commonly used, even though it can be affordable because many isotopes are produced at a time

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12
Q

What physical properties should a radiopharmaceutical have?

A

The RP must show suitable in vivo behaviours, which are influenced by its:
o Redox properties
o Stability
o Stereochemistry
o Charge
o Lipophilicity

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13
Q

In terms of organ/issue specificity, what are the mechanisms of localisation?

A

• Active transport
• Simple diffusion
• Capillary blockade
• Phagocytosis
• Cell sequestration
• Compartmental localisation
• Antigen-antibody complexation

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14
Q

In terms of RP used for diagnostics, RP are dectected using what?

A

Gamma scintigraphy: RP emits gamma (g) radiation. Detected by a gamma camera and/or SPECT (single-photon emission computed tomography)

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15
Q

Why is it importnat that the energy of the decaying gamma ray photon is within the specific energy window of the camera?

A

As gamma decay energies outisde the range of 100-250 keV (150 keV is optimal) will produce low quality images.

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16
Q

How are RP detected in gamma cameras?

A

• Positron (β+) decay results in the emission of two 511 keV gamma ray photons 180°apart
• Outside the gamma camera energy window
• Detected by a PET (positron emission tomography) camera
• PET scanners contain a circular array of detectors designed to specifically detect 511 keV photons emitted in opposite directions

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17
Q

What radioisotopes are used for the assessment of Myocardial Ischemia (perfusion studies)?

A

201Tl(Tl chloride) and 99mTc(Tc isonitriles)

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18
Q

How does 201Tl work as a marker in cardiac imaging?

A

• As an analog of K+, 201Tl acts as a marker of myocyte permeability
• Actively transported into cells by the Na2+/K+ pump

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19
Q

What are the disadvantages of 201Tl as a marker in cardiac imaging?

A

• Low energy emission range: 69-83 keV
• t1/2 = 73 h
• Cyclotron-produced

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20
Q

What is the most widely used RP in nuclear medicine?

A

99mTc which is a radiometal

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21
Q

What are the benefits of 99MmTc agents?

A

• Emits gamma rays at 140 keVoOptimal for gamma camera imaging
• Less scattered radiation than Tl; clearer images
• t1/2 = 6 h (can administer 10-15 times higher dose than Tl)
• 99mTc produced by generator and thus available on site

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22
Q

What radioisotopes are used in the assessment of myocardial necrosis?

A

[111In]-labelled antimyosin and [99mTc]-labelled glucarate

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23
Q

How is [111In]-labelled antimyosin used in the assessment of myocardial necrosis?

A

• Breach of the sarcolemma exposes intracellular myosin heavy chain
• Antimyosin antibody specifically binds to this exposed protein
• Intravenous injection of radiolabelled antimyosin antibody readily detects areas of irreversible damage

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24
Q

How is [99mTc]-labelled glucarate used in the assessment of myocardial necrosis?

A

• Glucarate uptake occurs in the first 9 h of acute myocardial necrosis
• Suitable for use as an emergency room marker
• This has replaced [99mTc]-labelled pyrophosphate, which gives a positive test 48-72 h after the acute event

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25
Q

What radioisotope is used in the assessment of myocardial inflammation?

A

[111In]-labelled leukocytes

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26
Q

How is [111In]-labelled leukocytes used in the assessment of myocardial inflammation?

A

• Myocardial cell damage and death produces an acute inflammatory response, accompanied by an influx of lymphocytes
• White blood cells are:
- Isolated from venous blood
- Tagged with 111In
- Injected back into the body
• 111In-oxine is the only labelling reagent that has FDA approval
• Although the GOLD STANDARD for this type of imaging, this technique has some disadvantages

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27
Q

What are the principles for brain imaging?

A

• Water-soluble radiopharmaceuticals are excluded from normal brain due to intact BBB (blood-brain barrier)
• Damaged brain shows up as an area of focal uptake

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28
Q

What are the indications/purpose of brain imaging?

A

• To screen patients for the presence of primary tumours
• To detect cerebral metastases
• To evaluate patients with cerebrovascular disease
• To detect intracranial injury
• To study patients with intracranial disease such as meningitis, encephalitis, neurodegenerative disease etc.

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29
Q

What are non-diffusible tracers in terms of brain imaging?

A

Ionised hydrophilic molecules that do not diffuse across the intact BBB

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30
Q

What non-diffusible tracers are commonly used in brain imaging?

A

99mTc-pentetate (DTPA) and 99mTc-gluceptate

31
Q

What are diffusible tracers in terms of brain imaging?

A

Neutral lipophilic agents that diffuse freely across the intact BBB

32
Q

What diffusible tracers are commonly used in brain imaging?

A

99mTc-hexamethyl propylene amine oxime (HMPAO; Ceretec®) and 99mTc-ethyl cysteinate dimer (ECD; Neurolite®)

33
Q

What happens when bone gets injured?

A

Increase in new bone formation and Increase in skeletal blood flow

34
Q

How do bone-seeking radiolabelled complexes aid in visualising bone injuries?

A

Bone-seeking radiolabelled complexes concentrate in skeletal lesions
as there is higher lesion/normal bone ratio (L/NB)
which allows for enhanced visualisation

35
Q

What is the most frequently used bone seeking agent used in bone imaging?

A

99mTc-methylene diphosphonate (MDP) Bone Sca

36
Q

What are the advantages of 99mTc-methylene diphosphonate (MDP) Bone Scan?

A

Detects lesions long before skeletal x-ray
• Fast blood clearance
• High skeletal affinity

37
Q

What are the purpose of thyroid imaging agents?

A

• To image solitary or multiple thyroid nodules
• To assess thyroid size and function
• Aid in the management of thyroid cancer

38
Q

How does thyroid imaging work?

A

• Thyroid traps and concentrates iodine from the blood
• Iodine is organically incorporated and stored as thyroglobulin
• Radiolabelled iodine: high concentration and prolonged storage in thyroid permits easy visualisation

39
Q

What radioisotopes are used for thyroid imaging?

A
  • (123I) NaI (oral administration; imaging at 24 h)
  • (125I) NaI
  • (131I) NaI
  • 99mTc-pertechnetate
40
Q

How does (123I) NaI work in thyroid imaging?

A

• Gamma emitter, therefore radiation is penetrating and short-lived (t1/2 = 13 h)
• Gamma energy: 159 KeV
• Diagnostic applications: imaging of the thyroid and thyroid metastases
• Produced using cyclotron: expensive

41
Q

How is (125I) NaI used in thyroid imaging?

A

• 125I gamma emitter of medium penetration (t1/2 = 60 days!)
• Gamma energy: 35 KeV
• Only used diagnostically when the test requires a longer period to prepare the radiopharmaceutical and trace it

42
Q

How is (131I) NaI used in thyroid imaging?

A

• b (beta decay) and gamma emitter (t1/2 = 8 days)
• 131I is not used for imaging alone, due to high exposure from bemissions which will destroy thyroid/thyroid cancer tissue

43
Q

How is 99mTc-pertechnetate used in thyroid imaging?

A

• May be used if imaging performed rapidly

44
Q

What two radioisotopes are used for providing information on kidney function?

A

99mTc-pentetate (DTPA) and 123I-o-iodohippuric acid [OIH] (Hippuran®) or99mTc-mercaptoacetyl triglycine (MAG-3®)

45
Q

How is 123I-o-iodohippuric acid [OIH] (Hippuran®) or99mTc-mercaptoacetyl triglycine (MAG-3®) used in assessing kidney function?

A

• Hippuric acid normal constituent of human urine
• Determination of renal perfusion/urine production

46
Q

How is 99mTc-pentetate (DTPA) used to assess kidney function?

A

• “Pure” glomerular agent
• Concentration reaches a peak 3-4 min post-injection
• At 2 h, 50% excreted in urine; 95% by 24 h

47
Q

What radioisotope is used for architectural imaging of the kidney?

A

99mTc-gluceptate

48
Q

How is 99mTc-gluceptate used for architectural imaging of the kidney?

A

• Localises to the renal cortex - useful for “architectural” imaging
• Detection of tumours or cysts, information on kidney size and position

49
Q

What are the benefits of RP in imaging? (5 points)

A

• Superior sensitivity and specificity
• Unique information; e.g. function or perfusion of specific tissues
• Labelling techniques are simple, robust and well-established
• Non-radioactive methods involve bulky chemical labels which can interfere with the process being investigated
• Quantifiable

50
Q

What is the primary use of RP in therapeutic applications?

A

Primarily directed against cancerous tissues utilising “the concept of tissue radiosensitivity”

51
Q

What is the goal of RP use in therapeutic applications?

A

To destroy diseased or cancerous tissue whilst sparing adjacent healthy tissue

52
Q

How do we choose an appropriate RP for a specific therapeutic use?

A

• Must have a high affinity for the diseased tissue
• Type and energy of radiation and its range in tissue are of prime importance
• Normally given orally or intravenously
• Uptake tells how much, or what %, of an administered radiopharmaceutical will actually be:
o Absorbed
o Metabolised
o Stored in the target organ

53
Q

How are beta minus emitters used in therapeutic applictaions? How are beta minus emitters used in therapeutic applictaions?

A

• High energy but limited range in tissue
• Deposits all its energy in the immediate vicinity of the organ
• Deposition is measured in gray (Gy) or rad
• Dose to the organ given in Gy/MBq or rad/mCi
• Radiation destroys the cells in the target organ

54
Q

How is 131I: NaI Therapy used in therapeutic applictaions?

A

• Administered orally for both Graves Disease - high uptake (hyperthyroidism) and thyroid carcinoma - low uptake
• 131I may come encapsulated in gel caps (expensive) or as a liquid (inexpensive)

55
Q

How is High Dose 131I Therapy used in therapeutic applications?

A

• 131I: 8 day effective half-life; emits beta- particles and gamma rays
• Patient is counselled regarding personal actions before they leave
• Patients who receive more than 1220 MBq are hospitalised until the quantity of radioactive material in their body drops to below 1220 MBq
• Generally these patients have had their thyroid surgically removed
• Destroys any residual thyroid tissue or metastatic thyroid carcinoma
• Patients should be monitored for signs of bone marrow suppression

56
Q

What dose is used in 131I: NaI Therapy for Graves disease?

A

Dose 370-1110 MBq (10-30 mCi)

57
Q

What dose is used in 131I: NaI Therapy for Thyroid Carcinoma?

A

Dose 925-5550 MBq (25-150 mCi)

58
Q

How is 131I-mIBG Therapy used in therapeutic applications?

A

• For the treatment of neuroendocrine tumours, such as neuroblastoma
• Structural analogue of guanethidine that has structural similarities to noradrenaline
• Is selectively taken up by adrenergic neurons, the adrenal medulla and some neuroendocrine cancer cells by an active uptake mechanism at the cell membrane
• Major toxic side effect is radiation-induced bone marrow suppression

59
Q

How is 32P Therapy used in therapeutic applications?

A

Treatment of some blood cancers:
• polycythaemia (increased red blood cells)
• essential thrombocythaemia (increased platelets)
Administered as sodium phosphate. After i.v. administration, 32P concentrates in blood cell precursors of the bone marrow where there is a rapid production of cells. Patient must be carefully monitored after treatment, as they have an elevated risk of leukaemia

60
Q

How is (32P, 89Sr, 153Sm, 186Re) in pallative therapy?

A

Relief of pain in:
o Metastatic bone cancer
o Other joint disorders (e.g. haemophilic arthropathy)
Injection of b-emitting bone-seekers spare the patient much pain and degradation by numbing the nerve endings within the bone. High-energy b-emitters are injected directly into joint

61
Q

What is a Radiopharmacy?

A

A laboratory suite designed for the preparation and dispensing of radioisotope-labelled pharmaceuticals for therapeutic and diagnostic purposes

62
Q

What does 99Mo/99mTc Generator allow for?

A

Allows for a short-lived daughter radionuclide to be separated from a longer-lived parent radionuclide

63
Q

What does 99Mo/99mTc generator provide?

A

The 99Mo/99mTc generator provides a rapid and highly efficient system for the production of 99mTc “in-house”

64
Q

In the eluate, what is the 99Mo-99mTc ratio strictly limited to?

A

0.15 mCi/1 mCi (0.015%)

65
Q

In what form does 99Mo/99mTc Generator produce 99mTc?

A

In the form of sodium pertechnetate (+7)

66
Q

Where is the parent (99MoO42-) adsorbed onto?

A

The Parent (99MoO42-) is adsorbed onto a sterile ion-exchange column, commonly aluminium trioxide (alumina)

67
Q

What is the half life (t1/2) of 99Mo?

A

99Mo decays gradually (t½ = 66 h)

68
Q

How often is the generator milked (eluted)?

A

Around once daily

69
Q

How many oxidation states does Technetium potentially have?

A

Technetium has 9 potential oxidation states

70
Q

What type of radioisotope is 2-[18F]fluoro-2-deoxyglucose (FDG)?

A

It is a positron (ß+) emitting radioisotope

71
Q

When does 99mTc reach is maximum in the generator?

A

After about 24 hours

72
Q

What can 2-[18F]fluoro-2-deoxyglucose (FDG) used for?

A

It can be used to distinguish viable, but functionally impaired, ischemic myocardium from nonviable (infarcted) myocardium

73
Q

How does 2-[18F]fluoro-2-deoxyglucose (FDG) work in terms of diagnistic imaging?

A

FDG is moved into cells by glucose transporters and is then phosphorylated by HK to FDG-6- phosphate. FDG-6- phosphate cannot be metabolized further in the glycolytic pathway and stays intracellularly in the cells.

PMY2006 (12 decks)

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