RADIOPHARMACEUTICALS

UNIT - 5TH

SYLLABUS :

Radiopharmaceuticals: Radio activity, Measurement of radioactivity, properties of $\alpha$, $\beta$, $\gamma$ radiations, Half life, radio Isotopes and study of radio isotopes sodium Iodide $I^{131}$, storage conditions, preparations & pharmaceutical application of radio active substances.

Radiopharmaceuticals

These are radioactive substances which are used therapeutically for treatment and diagnosis.

• these substances get distributed in the body and emit radiations, which used to detect injury, abnormal cells growth etc..
• when these rays passes, they penetrate the body tissue, muscle but not bones which gives x-ray images, helps to identified injury & fractures etc.

Radioactivity

It is phenomena in which radioactive substances or radioisotopes [which are unstable] undergoes decomposition or decay by emission of radiation/rays to get stable.

• radiations are emitted from the nuclei of element.
• there are various types of radiations, but the most common are alpha, Beta and Gamma.
• A branch of Pharmacy which deals with radioactive substances are known as Radiopharmacy.

RADIATION

Radioactive substances emitted radiations which is travel through a medium/space and absorbed by another medium.

1. Alpha rays ($\alpha$-rays]:

• These are positively charged particles and heavy.
• They have the least penetrating power.
• These particles are equivalent to the nuclei of helium atoms (i.e. $He^{2+}$).
• They are highly energetic particles and have energy upto $9 MeV$. $$^{226}Ra \rightarrow ^{222}Rn + ^{4}_{2}He (\alpha)$$
• less/not useful in p'ceutical formulation.

2. Beta rays/radiation [$\beta$-rays]:

• They are mostly negative charged particles (negatron or electron), but rarely may also be positively charged. [positron]
• They have great penetrating power than that of the $\alpha$-rays.
• Their mass is equal to an electron [approx. $\frac{1}{1800}$ of unit mass] $$^{14}{6}C \rightarrow ^{14}{7}N + \beta + e^{-}$$
• The maximum energy exhibited by beta particles (E_{max}$) is $1.5 MeV and mean energy (\bar{E}$) is $0.6 MeV.

3. Gamma radiation [$\gamma$-rays]

• Gamma particles are similar to X-radiation.
• It is an electromagnetic radiation, therefore does not have any mass or charge. It is neutral.
• They have more penetrating power than alpha and beta rays, and travel with the speed of light.
• Gamma particles are high energy particles, typically $2 MeV$.

APPLICATIONS OF RADIATIONS

• Radiations are used for diagnostic purposes. x-rays.
• used in sterilization as they destroy bacteria & other microbes.
• various radioactive isotopes are used to supply power to satellites and provide electricity for space laboratories.
• they are utilised for estimating the degree of air pollution.

UNITS OF RADIOACTIVITY

It is the no. of disintegrations per second.

1. Curie ($Ci$) : *

• It is the conventional unit.
• One curie is defined as activity of material which has $3.7 \times 10^{10}$ disintegration per second.

$1 \text{ curie} = 3.7 \times 10^{10} \text{ dps or } (37 \text{ trillion decay}), Bq.$

2. Becquerel ($Bq$): It is SI unit. $1 Bq = 2.703 \times 10^{-11} Ci$ [1 disintegration per second].

3. Roentgen ($R$): International system of units etc.. $1 R = 2.58 \times 10^{-4} C kg^{-1}$ [$C$ = Coulomb]

Decay Constant

It is the actual amount of emission of radiation/ray per unit time.

where, $\lambda =$ radioactive decay or disintegration constant. $N =$ quantity, $t =$ time.

Half-life

It is the time period in which a substance or radionuclide is reduced by $50\%$ or half of its initial amount.

where, $\lambda$ is disintegration constant in unit of $sec^{-1}$.

• Iodine-131 $\rightarrow$ 8 days
• Polonium-212 $\rightarrow$ $3 \times 10^7$ seconds
• Uranium-238 $\rightarrow$ $4.5 \times 10^9$ years etc..

MEASUREMENT OF RADIOACTIVITY

The radioactivity of alpha, beta and gamma can be measured by various techniques :-

1. Ionisation chamber
2. Geiger-Muller counter
3. Scintillation counter

These techniques mainly work by detection and counting of radiations.

1. Ionisation Chamber

• It simply measure the radiation strength by measuring the strength of current.

$$Total \ amount \ of \ charge/current \propto Total \ amount \ of \ radiations$$

• Ionisation chamber is filled with two metallic plates, separated by air.
• on passing the radiation through the chamber, the atoms of air molecules are knocked off and the charged formed.
• The electrons (-ve) move towards the anode while positive ions towards the cathode, which formed the current measured in ammeter.
• The total amount of charge passing b/w the plates in a given time is measured by dosimeter, present in ionisation chamber.

2. Geiger-Muller counter

The rate of emission of $\alpha$ or $\beta$-particles can be detected and measured by this geiger-muller counter.

• It comprises of a cylindrical metal tube which acts as cathode, and for anode a wire is placed carefully.
• The metal tube has very low pressure of around $0.1 atm$ and filled with argon gas.
• A potential difference of 1000 volts is maintained across electrodes.
• Radiations enters through mica window and argon gas ionised and $Ar^+$ move towards cathode while electro move towards anode.
• The circuit is completed in very short time (in microseconds), after which an electric pulse is generated which is recorded in the automatic counter.
• The intensity of radioactivity is measured by the pulse generated per minute.

3. Scintillation counter

It works on detecting a flash or scintillation of light, which is produced when a charged particle, or X-rays & gamma rays, strikes on either fluorescent screen.

• It is based on that the radiation enter is strikes on photocathode, which then multiply through dynodes [all anodes] and collected at output.
• then this output pulse is measured, which is directly proportional to the intensity of radiations.

RADIOISOTOPES

A radioisotope is an atom with an unstable nucleus i.e. an unstable combination of neutrons and protons. Now, these atoms emitted radiations/rays to make itself stable.

• As a isotopes they have same no. of proton but varies neutrons.

Types :

1. Stable radioisotopes: these are stable and do not emit radiations.
   $^{12}C$, $^{13}C$, $^1H$ (protium), $^2H$ (deuterium) etc..

2. Radioactive radioisotopes: they are either naturally or artificially produced unstable isotopes. so, they emit radiations to loose energy. phenomena is known as radioactivity.
   $$^{14}_6C \rightarrow ^{14}_7N + \beta$$

Study of Radioisotopes :

1. Carbon element : It has three isotopes

2. Iodine element : It has two isotopes

   • Iodine-127 $\rightarrow$ 53 protons, 74 neutrons $\rightarrow$ stable isotope
   • Iodine-131 (^{131}_{53}I$) $\rightarrow 53 protons, 78 neutrons, unstable isotope.

3. Phosphorus element : It has two isotopes:-

   • phosphorus-31 (^{31}_{15}P$) $\rightarrow stable
   • phosphorus-32/32P (^{32}_{15}P$) $\rightarrow Unstable

PRECAUTIONS IN HANDLING & STORAGE OF RADIOP'CEUTICALS

• Great care has to be taken in handling and storage of radioactive material for protecting people and personnel handling it.
• One should not touch radioactive emitter with hand but it should be handled by means of forceps or suitable instruments.
• While handling liquid radioactive materials, rubber gloves should be used.
• The radioactive material should not contaminate the working area.
• Smoking, eating and drinking activities should not be done in the laboratories where the radioactive material are handled.
• Sufficient protecting clothing and shielding devices have to be used while handling the materials.
• Radioactive substances are suggested to be used within a short period time.
• The disposal of radioactive material is done with great care.

STORAGE :

• The radiopharmaceuticals are stored in airtight containers in a shielded place, and in suitable labelled containers.
• Stored place should be monitored as per National/International regulations for radioactive substances.
• The containers of radiopharmaceuticals may become dark due to irradiation. & optimum storage condition should be maintained.
• All the requirements are prescribed by the department of atomic energy [DAE] for the establishment of a radio-isotope facility in a hospital or pharmacy.

PRODUCTION OF ISOTOPES/RADIOISOTOPES

a) Reactor nuclear irradiation : A reactor is having a moderator, which slow down the fast neutrons to thermal energies.

b) Cyclotrons reactors:- Radionuclides stable by positron emission or nuclei, therefore resultants.

Therapeutic Applications :

• These radiations have destructive features, so destroy abnormally multiplied cells and further inhibit the formation of new cells and tissue.
• Used in the treatment of disorders like cancers which involves cellular malfunction.
• (e.g. Iodine-131 [sodium iodide] used for thyroid scanning. diagnosis of various organs imaging. e.g. for myocardial)

3. Sterilization :

• Radiations are used to sterilise the pharmaceutical and surgical instruments in hospitals.
• Cerium-137 may be used for sterilising surgical instruments.

4. Research :

• Excellent biological and medicinal studies have been carried out with radio-active isotopes as tracers.
• e.g. $^{14}C$ and $^3H$ are most commonly used radio-nuclides for this.

SODIUM IODIDE $I^{131}$

It is a radiopharmaceutical substance used in the treatment of malignant thyroid. The ionising radiations of Iodine $I^{131}$ are absorbed by the thyroid tissue and damage that tissue.

• The sodium iodide $I^{131}$ emits about $90\%$ of \beta$-radiations and the remaining $10\% is the $\gamma$-radiation.

Mechanism : The iodide enters into thyroid through the sodium/iodide symporter and accumulates there. Here it oxidises into iodine and emits radiations. Now, these $\beta$-radiations destroy the thyroid tissue.

Properties :

• It is a colorless solution having a pH between $7-10$.
• Half-Life of sodium iodide $I^{131}$ is $8.4$ days.
• It emits $\beta$- and $\gamma$-radiations.

Uses :

• It is used in the treatment of hyperthyroidism and some cases of thyroid malignancy.