Radiochemistry & Radiopharmaceuticals

1. Radiochemistry and
Radiopharmaceuticals
Prachi Pathak

2. Contents
Fundamentals of radioactivity:
• Properties of radionuclide, Radionuclide, Radioisotope, Radioactive decay,
halflife of radioactivity, specific activity, Becquerel, curie, Sievert and
Gray(Self
study-0.5 h)
• Relative biological effectiveness, Radionuclidic purity, Radiochemical
purity, Geiger-Muller Counting, liquid Scintillation Counting
• Safety aspects of radiopharmaceutical laboratory (Self study-0.5 h)
Quality control of radiopharmaceuticals: Physical, Chemical (Radionuclidic
purity, Radiochemical purity), and pharmaceutical properties (Self study-0.5
h-apyrogenicity, pH and absence of particulate), Isotope dilution analysis
(Direct and Inverse), 99mTc generator.

3. Nuclide
 All atomic nuclei are made up of a collection of protons and
neutrons. (Except for hydrogen, which consists of a proton only)
 The chemical properties of an atom are determined by its
atomic number Z, which is the number of protons contained in
its nucleus.
 A nuclide is characterized by the number of protons and
neutrons in the nucleus.
 The sum of the number of neutrons and protons in a nucleus
is the mass number.

4. Radionuclide
A radionuclide is a nuclide which is radioactive i.e.
radiation emitted by spontaneous transformation
of the nucleus.

5. Isotopes
 Isotopes are nuclides of the same element
having same atomic number(Z) but different
mass numbers(A). i.e, the nuclei of isotopes of
an element contain the same number of
protons but different numbers of neutrons.

6. Radioisotope
• Radioisotope, any of several species of the
same chemical element with different masses
whose nuclei are unstable and dissipate
excess energy by spontaneously
emitting radiation in the form of alpha, beta,
and gamma rays.

7. Radioisotope
• Every chemical element has one or more
radioactive isotopes.
• For example, hydrogen, the lightest element,
has three isotopes with mass numbers 1, 2,
and 3.
• Only hydrogen-3 (tritium), however, is a
radioactive isotope, the other two being
stable.

8. Radioactivity
Transformation of radionuclide may involve
emission of charged particles (α and β particles)
which may be accompanied by the emission of
γ radiation.

9. Radioactive Decay
 It is a random first order process independent of
temperature and the chemical state of the
radionuclide.
 Fundamental Radioactive decay law is
A – activity at time t
A0 – initial activity
λ – Decay constant
A = A0e-λt

10. Decay Constant
• Number of atoms decaying per unit time is
proportional to the number of unstable atoms
• Constant of proportionality is the decay
constant ()
-dN/dt =  N
A =  N
10radiopharmaceuticals

11. Half-Life of Radioactivity
• The Half-Life of a radionuclide is defined as the time
for the activity to decay to one half of its initial value.
A0/2 = A0e-λt
½ = e-t1/2
ln 2 = λt1/2
λ = ln 2/ t1/2
11
A = A0e-λt
λ = 0.693
t1/2

12. Half-Life of Radioactivity
• Half-life and decay constant are inversely related
and unique for each radionuclide
• Thus in two half lives the activity has fallen down
to 1/4 its initial activity.
• i.e. in 10 half lives activity falls to 10-3 of its initial
value(0.1% of initial activity)
• After 20 half lives activity will have fallen to 10-6
of initial value.
λ = 0.693
t1/2

13. Half-Life
13

14. 14
Half-Life

15. Specific activity
• The radioactivity per unit mass of material, is
called specific activity (A)
• The basic unit for quantifying radioactivity
(i.e. describes the rate at which the nuclei
decay).
• Unit: curies or Bequerel.

16. Units : Becquerel , Curie , Gray
• Becquerel (Bq): A unit of radioactivity. One
becquerel is equal to 1 disintegration per
second.
• Curie (Ci): A unit of radioactivity. A curie is
defined as 3.7 x 1010 disintegrations per second.
• The unit of absorbed dose is gray (Gy) defined
as the generation of 1 J/Kg

17. Sievert
• Sievert is the unit of absorbed dose equivalent and equal to
100 rem.
Rad - radiation absorbed dose is a more universal unit, it is a
measure of the energy deposited in unit mass of any material
by any type of radiation.
The dose equivalent unit (rem) roentgen equivalent man
Rem has been developed to account for the differences in
effectiveness of different radiations in causing biological
damage.
In radiobiology, the rem is defiend as
Rem= rad  RBE
RBE is the relative biological effectiveness of the radiation.

18. Relative Biological Effectiveness (RBE)
• The relative biological effectiveness is the
ratio of biological effectiveness of one type
of ionizing radiation relative to another, given
the same amount of absorbed energy.
• The RBE is an empirical value that varies
depending on the particles, energies involved,
and which biological effects are relevant.

19. Radionuclidic purity
• The radionuclide purity of a preparation is
that percentage of the total radioactivity that
is present in the form of the stated
radionuclide.

20. Radiochemical purity
• The radiochemical purity of a preparation is that percentage
of the stated radionuclide that is present in the stated
chemical form.
• As radiochemical purity may change with time, mainly
because of radiation decomposition, the result of the
radiochemical purity test should be started at given date and
if necessary hour indicating when the test was carried out.
• The radiochemical purity limit should be valid during the
whole shelf-life.

21. Measurement of radioactivity:
Geiger muller counting
• Ability of radiation to cause ionisation of gas
• Easily ionisable gas is argon
• Beta particles enters the Geiger muller tube
through thin mica window causes some
argon atom to (excite) ionise.
• Ionised argon ions are attracted towards
outer -ve electrode by substantial potential
gradient.
• Further ions are formed due movement of
heavy beta charged ions which are attracted
to anode causes further ionisation.
• This results in ionisation of whole volume
present in detector tube which is equivalent
to flow of pulse of current output 21

22. Geiger muller counting
• These charges are collected by the anode
and cathode which then form a very small
current in the wires going to the detector.
• By current measuring device between the
wires from the cathode and anode, the
small current is measured and displayed as
a signal. More the radiation enters the
chamber, more is the current displayed by
the instrument.
• All the pulses produced are of same value
irrespective of the energy of the beta
particles initiating this effect. i.e.
nonproportional counter.
• Efficiency is not high for low energy
emitters and not possible to detect 3H.
22

23. Geiger muller counting
23

24. Geiger muller counting

25. Liquid scintillation counter
• The measurement of low energy Beta emitters egs.
3H and 14C which have wide applications in tracer
work eg. Drug metabolism is carried out by Liquid
scintillation counting.
• Sample is dissolved in solvents(toluene or dioxane
but not water) together with some scintillant
material like diphenyloxazole (PPO) .
• This Scintillant molecule can be raised to excited
electronic state from which it falls to ground state
with the emission of visible light i.e. fluorescence.
25

26. Scintillation
• The term scintillation is used to cover the
whole sequence of events whereby energy of
fast moving electron (the emitted beta
particles) is transferred via the solvent to the
scintillant molecules which are thereby raised
to the excited electronic level and then
fluoresce with the emission of light which is
detected by photomultipliers.
26

27. • The first part of the photomultiplier
tube is made of another special
material called a photocathode.
• The photocathode produces electrons
when light strikes its surface.
• These electrons are then pulled
towards a series of plates called
dynodes through the application of a
positive high voltage.
• When electrons from the photocathode
hit the first dynode, several electrons
are produced for each initial electron
hitting its surface. 27
Liquid scintillation counter

28. Liquid scintillation counter
• This “bunch” of electrons is then pulled
towards the next dynode, where more
electron “multiplication” occurs.
• The sequence continues until the last
dynode is reached, where the electron
pulse is now millions of times larger then it
was at the beginning of the tube.
• At this point the electrons are collected by
an anode at the end of the tube forming an
electronic pulse. The pulse is then detected
and displayed by a special instrument.
• Scintillation detectors are very sensitive
radiation instruments and are used for
special environmental surveys and as
laboratory instruments.

29. Liquid scintillation counter
29

30. Liquid scintillation counter
30

31. Liquid scintillation counter
31radiopharmaceuticals

32. Safety Aspects of
radiopharmaceutical
Laboratory

33. Classified Workers
• Are those who are likely to receive a dose of ionising
radiation which exceeds 3/10 of the dose limit. This is set at
50 mSv per annum.
General public
• The dose limit is set at 5 mSv per annum.
Controlled area
• If radiation doses exceeding the 3/10 dose limit are likely to
be received by workers i.e 15 mSv per annum.
Supervised areas
• Area where annual dose is likely to exceed 5 mSv
According to Ionising Radiations Regulations 1985-

34. All uses of radioactive materials must
be based on three general principles :
• Every activity resulting in exposure to ionising
radiation shall be justified by the advantages it
produces
• All exposures shall be kept as low as is reasonably
achievable
• Doses received shall not exceed certain, defined
limits.

35. Practical Radiation Protection
• With alpha emitters and low energy beta emitters (e.g
3H, 14C) contamination is the major hazard.
• Regulations within radiopharmacy area must be on –
- ingestion, inhalation or contamination cannot occur
- smoking, drinking, eating , pipetting by mouth must be
forbidden
- Gloves must be worn while handling radioactive
material
- operations must be performed in fume hoods
- Personnel must wash hands and monitor themselves on
leaving the area.

36. • With gamma emitters, in addition to avoiding any
form of contamination there is also the radiation
hazard.
• The three key factors in minimising the radiation
dose received are time, distance and shielding.
Practical Radiation Protection

37. • The time in performing the operation should be
minimised
• The practice with non radioactive material is
invaluable here.
Time
• Distance between the radioactive source and the
operator should be maximized – by use of forceps and
tongs
• Radiation flux α 1/ (distance)2 . Large reductions in
received dose can be achieved. (if distance is doubled
dose received is reduced to 1/4)
Distance
• Normally using lead bricks
• To reduce the dose rate at the operator’s body to less
than 7.5µSv h-1
Shielding
Practical Radiation Protection

38. Basic laboratory Organisation
• All floor coverings should consist of a single sheet of
non absorbent material; gaps and joins should be
avoided; if unavoidable, joins should be sealed with a
suitable sealant.
• Additional coating of non-absorbent, non permeable
coating, e.g polyurethane
• Walls and ceiling should be covered with a non-
absorbent, gloss coating , preferably strippable for ease
of removal in the event of contamination.

39. Basic laboratory Organisation
• Bench tops should be of non absorbent material
(formica not wood).
• Joints to walls should be sealed. All the edges
should be sealed
• Fume hoods should be utilized wherever possible
• Air flow and venting of such fume hoods requires
specialist advice

40. Basic laboratory Organization
• The counting room should be separate from the
radiochemical laboratory but adjoining to it
• Only samples and sources appropriately prepared
and contained for counting should be taken into
counting room.
• Good manufacturing practice and aseptic handling
facilities should be maintained
• Safety aspects for handling of large amounts of
99mTc should be considered.

41. Quality Control of
Radiopharmaceuticals
• Radiopharmaceuticals they undergo strict
quality control measures
• Involves several specific tests and
measurements that ensure
– Purity
– Potency
– Product identity
– Biologic safety
– Efficacy

42. Physicochemical Tests
• Tests that are essential for the
determination of the purity and
integrity of a
radiopharmaceutical.
• Some of these tests are unique
for radiopharmaceuticals
because they contain
radionuclides.

43. Physical Characteristics
• Colour and state of a radiopharmaceutical
• A true solution should not contain any
particulate matter
• Any deviation from the original colour and
clarity should be viewed with concern because
it may reflect changes in the
radiopharmaceutical that would alter its
biologic behaviour

44. • Colloidal or
aggregate
preparations should
have a proper size
range of particles for
a given purpose
Physical Characteristics

45. pH and Ionic Strength
• All radiopharmaceuticals should have
an appropriate hydrogen ion
concentration or pH for their stability
and integrity
• Ideal pH of a radiopharmaceutical
should be 7.4
• pH can vary between 2 and 9 because
of the high buffer capacity of the
blood

46. pH and Ionic Strength
• Measured by a pH meter
• Radiopharmaceuticals must also have proper
– Ionic strength
– Isotonicity
– Osmolality
in order to be suitable for human administration
• How to achieve Correct ionic strength?
• By adding a proper
– Acid
– Alkali
– Electrolyte

47. Apyrogenicity
• All radiopharmaceuticals required for human
administration should be pyrogen free.
• Pyrogens are either polysaccharides or proteins produced
by metabolism of microorganisms
• They are over 0.05-1mm in size, soluble and heat stable
• Following symptoms are produced by administration of
pyrogens –
Fever, chills, leukopenia, pain in joints, flushing, sweating,
headache and dilation of pupils.

48. Pyrogenicity Testing
USP-Rabbit test
LAL (Limulus amebocyte lysate) test
• The principle of the test is based on the
formation of opaque gel by pyrogens upon
incubating the sample with LAL at 37oC
• An assay mixture usually consists of 0.1ml LAL
and the test sample at pH 6-8
• The reaction takes place in 15-20min after
mixing and depends upon concentration of
pyrogens.

49. Radionuclidic Purity
• Defined as
– The percentage of the total radioactivity due to
specified radionuclide present in a
radiopharmaceutical
• Impurities arise from
– Extraneous nuclear reactions due to isotopic
impurities in the target material
– Fission of heavy elements in the reactor

50. Radionuclidic Purity
• The undesirable radionuclides may belong to
the same element as the desired radionuclide
or to a different element
• Impurities can be removed by appropriate
chemical methods
• Radionuclidic purity is determined by
measuring the half-lives and characteristic
radiations emitted by individual radionuclides

51. • Examples:
• Sodium iodide 125I should not have 126I present
to an amount greater than 1%.
• Sodium pertechnetate 99mTc Injections should
not have 99Mo present to an amount greater
than 0.1%.
Radionuclidic Purity

52. • The standard method of determining
radionuclidic Purity is by gamma
spectrometry, whereby the characteristic
gamma ray emissions of radionuclides can be
used to -
- Detect &
- Quantify impurity levels.
Radionuclidic Purity

53. Principle of Gamma spectrometry
• It identifies unknown gamma radionuclides
and its use to determine the radionuclidic
purity of sodium iodide 125I.
• If 99mTc is available a rapid test for 99Mo in
99mTc is based on – The 140keV gamma proton
is entirely absorbed by a 6mm thickness of
lead where as 740keV gamma proton of 99Mo
is essentially attenuated.

54. Radiochemical Purity
• Defined as
• The percentage of the total radioactivity in the
specified chemical form in the
radiopharmaceutical
• Radiochemical impurities arise from
• Decomposition due to the action of solvent
• Change in temperature or pH, light
• Presence of oxidizing or reducing agents

55. Radiochemical Purity
• Example :
• A Radiochemical impurity in sodium iodide
125I would be iodate ion 125IO3
• The radioactivity associated with a chemical
form ; iodate is different from the specified
chemical form iodide.

56. Radiochemical Purity
• The Decomposition of radiopharmaceutical by
radiolysis occurs, where H. and OH. Free radicals
are produced by the interaction of emitted
radiation with water attack the radioactive
material, producing other chemical species.
• The very small masses of radioactive materials
involved make this problem of significant
proportions

57. Radiochemical Purity
• The stability of a compound is time-dependent
on exposure to light, change in temperature, and
radiolysis
• The longer a compound is exposed to these
conditions, the more it will tend to break down
• How to lessen the degradation of the material?
• Substances such as sodium ascorbate, ascorbic
acid, and sodium sulfite are often added to
maintain the stability of radiopharmaceuticals
• Some radiopharmaceuticals are stored in the
dark under refrigeration

58. Radiochemical Purity
• Analytical methods used to detect and determine the
radiochemical impurities in a given radiopharmaceutical:
– Precipitation
– Paper and Thin-Layer Chromatography
– Gel Chromatography
– Paper or Polyacrylamide Gel Electrophoresis
– Ion Exchange
– Solvent Extraction
– High-Performance Liquid Chromatography
– Distillation

59. Isotope Dilution Analysis (IDA)
• Isotope dilution analysis is the homogeneous
dilution of a labeled compound of known
specific activity with the same unlabelled
compound.
• The application of IDA results from the fact
that the compound need not be isolated
quantitatively. It is necessary only to isolate a
pure sample of the compound.

60. Direct IDA
• A known weight of labeled compound of known
specific activity is added to a mixture which
contains an unknown amount of same unlabelled
compound.
• Ao – Original activity of added labeled compound
• Mo -- Original amount of added labeled compound
• So -- Original specific activity of added labeled
compound
So = Ao/Mo

61. • Let Mu be the amount of unlabelled
compound present.
• S1 be the specific activity of the final isolated
sample.
S1 = Ao/ Mo+ Mu
Substituting for Ao (A0=SoMo)
S1 = SoMo/ Mo+ Mu
Solving for Mu
Mu = (So/S1 - 1) Mo
Direct IDA

62. Inverse or Reverse IDA
• A known quantity of non labeled compound is
added to a mixture containing an unknown
amount of labeled compound.
• Inverse method allows the addition of a
relatively large quantity of unlabelled carrier
compound; this facilitates isolation and
purification.
• This is especially useful in metabolism studies
of labeled drugs

63. 99mTc Generator
• Generators are based on Radioactive
Equilibrium .
• If one radionuclide decays to give another
radioactive material, then a radioactive
equilibrium can be established when the ratio
of the two half lives is appropriate.
• Eg:

64. 99mTc Generator
• A radioactive equilibrium will be established
whereby the rate of formation of 99mTc (from
the decay of 99Mo) will be equal to the rate of
decay of 99mTc (by isomeric transition to 99Tc)

65. 99mTc Generator
Initially
with
pure
99Mo
there is
no
99mTc
99mTc is
formed
by the
decay
of 99Mo
&
builds
up
99mTc is
decaying
to 99Tc via
gamma
emission
After 24hr
an
equilibrium
is
established
Both
99mTc
and
99Mo
decay
with
the
decay
constant
of the
parent
99Mo

66. 99mTc Generator
• Parent 99Mo and daughter 99mTc have different
chemistry can be utilized to produce the
‘generator’ which allows short lived 99mTc to
be produced daily in the hospital radio
pharmacy.

67. 99mTc Generator
• Most commercial 99Mo/99mTc generators use column
chromatography, in which 99Mo in the form of
molybdate, MoO4
2− is adsorbed onto acid alumina
(Al2O3).
• When the Mo99 decays it forms pertechnetate TcO4
−,
which, because of its single charge, is less tightly bound
to the alumina.
• Pouring normal saline solution through the column of
immobilized 99Mo elutes the soluble 99mTc, resulting in
a saline solution containing the 99mTc as the
pertechnetate, with sodium as the counterbalancing
cation.

68. A system for holding the parent in such a way that the daughter can be easily
separated for clinical use is called a radionuclide generator

69. Principle of 99mTc Generator