Radiopharmaceuticals

Radiopharmaceuticals are a group of pharmaceutical drugs containing radioactive isotopes. These compounds serve both diagnostic and therapeutic purposes. Radiopharmaceuticals emit radiation themselves. Radiopharmacology is the specialized branch of pharmacology that deals with these agents. The primary group of radiopharmaceuticals includes radiotracers, which are used to diagnose tissue dysfunction. Radiopharmaceuticals are administered to patients via injection or orally. They can be monitored and analysed using external medical devices and tests. In this article we will see Radioactivity, Measurement of radioactivity, Radiations, Half-life and Radioisotopes.

Table of Contents

Radioactivity

Radioactivity is the property exhibited by certain types of matter of emitting energy and subatomic particles spontaneously. It is, in essence, an attribute of individual atomic nuclei. An unstable nucleus will decompose spontaneously, or decay, into a more stable configuration but will do so only in a few specific ways by emitting certain particles or certain forms of electromagnetic energy. Radioactive decay is a property of several naturally occurring elements as well as of artificially produced isotopes of the elements.

Half life

The rate at which a radioactive element decays is expressed in terms of its half-life; i.e., the time required for one-half of any given quantity of the isotope to decay. Half-lives range from more than 10²⁴ years for some nuclei to less than 10⁻²³ second. Half life of any radioactive element is calculated by using following formula.

t/2= 0.696/λ

Where, λ is a decay constant. Half-life of some radioactive elements is given in table below.

Radioactive Element	Half Life (t/2)
²¹²Po	3×10^-7 seconds
¹³¹I	8 days
³²P	14.3 days
⁶⁵Zn	150 days
²²Na	2-6 years
²³⁸U	4.5×10⁴ years

Measurement of radioactivity

Radioactivity is determined by measuring the number of decay processes per unit time. The easiest way is simply to determine the number of counts per minute, with each count measuring a single decay process, such as the emission of an α-particle. Some more sophisticated methods are used for detecting and quantifying radioactivity are discussed below.

Methods Based Upon Gas Ionization

In ionization chambers, charged particles (such as α- and β-particles) dislodge orbital electrons from gas atoms, causing ionization.
The resulting current flow is proportional to the number of radiation particles entering the chamber.
Proportional counters, operating in the proportional region, are useful for detecting α-emitting isotopes.
Gas amplification via the Townsend avalanche effect enhances current flow.

Methods Based Upon Excitation

These methods involve detecting radiation-induced excitation of atoms or molecules.
Scintillation detectors use materials that emit light (scintillations) when excited by radiation.
Photomultiplier tubes amplify the scintillation light, allowing sensitive detection.
Solid-state detectors, such as semiconductor detectors, directly measure the energy deposited by radiation in the detector material.

Methods Based Upon Exposure of Photographic Emulsions

Photographic films or plates are exposed to radiation.
The radiation interacts with the emulsion, leaving tracks that can be developed and analyzed.
Historically used for detecting radiation, but now less common due to digital alternatives.

Radiations

Radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. The radiations emitted by radiopharmaceuticals are discussed below.

Alpha Particles (α)

Although less common, certain radiopharmaceuticals emit alpha particles.
Alpha decay releases helium nuclei (⁴/₂He).
Alpha emitters are primarily used for targeted cancer therapy.

Beta Particles (β)

Some radiopharmaceuticals emit beta particles.
Beta decay involves the emission of electrons (β⁻) or positrons (β⁺).
Beta emitters are used for both diagnostics and therapy.

Gamma Photons (γ)

Diagnostic radiopharmaceuticals emit gamma photons.
These high-energy photons can penetrate the body and are detected by external cameras.
Gamma imaging produces images used to visualize various organs, such as the brain, heart, and bones.

Positrons (β⁺)

Positron-emitting radiopharmaceuticals are used in positron emission tomography (PET).
Positrons annihilate with electrons, producing two gamma rays (511 keV each).
PET scans provide functional information about tissues and metabolic processes.

Properties of radiations

Radiations are emitted by atoms of radioactive material in the form of energy. This energy is of two types. One is particulate and another is electromagnetic. These two types are interchangeable. The particulate radiations are in the form of alpha and beta radiations emitted by disintegrating atoms of radioactive material. These are the beams of high speed charged particles and posses following properties.

Can be detected by electrical or magnetic field
Can penetrate matter in which it is travelling
Can ionise the matter
Cause certain substances to emit flashes of light (scintillation)
Can darken a photographic plate

Some of these properties are utilized in their detection and estimation or measurement. The ionizing effect is measured in ionisation chambers and Geiger Muller Counters, the scintillation effect in scintillation counters and the photographic effect in autoradiography.

Radioisotopes

Radioisotopes, also known as radioactive isotopes or radionuclides, are a species of the same chemical element with different masses whose nuclei are unstable. These unstable nuclei spontaneously emit radiation in the form of alpha, beta, and gamma rays. Here are some key points about radioisotopes:

Variety of Isotopes

Every chemical element has one or more radioactive isotopes.
For example, hydrogen has three isotopes: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium).
Tritium (hydrogen-3) is the only radioactive isotope among these.

Natural and Artificial Radioisotopes

Approximately 50 radioactive isotopes occur naturally.
The rest are artificially produced through nuclear reactions.
These artificial isotopes can be direct products of nuclear reactions or descendants of other radioactive products.

Chemical Behavior

When a radioactive isotope is added in small amounts to a stable element, it behaves chemically the same.
However, it can be traced using detection devices like Geiger counters.

Examples

Iodine-131: Used in treating hyperthyroidism.
Carbon-14: Used in breath tests to detect Heliobacter pylori bacteria causing ulcers.

Applications of radioisotopes

Radioisotopes find crucial applications in medicine, enhancing our understanding and improving patient care.

Diagnostic Imaging: Radioisotopes are used for medical imaging to visualize specific organs and tissues. Technetium-99m is widely employed in single-photon emission computed tomography (SPECT) scans. Iodine-131 is used for thyroid imaging and function assessment.
Positron Emission Tomography (PET): PET scans utilize positron-emitting radioisotopes. Fluorine-18 (F-18) is commonly used in PET imaging. PET provides functional information about metabolic processes and disease progression.
Radiotherapy: Radioisotopes are used for cancer treatment. Cobalt-60 emits gamma rays and is used in external beam radiotherapy. Iodine-131 is employed for targeted radiation therapy in thyroid cancer.
Cardiovascular Studies: Radioisotopes help assess heart function. Thallium-201 is used for myocardial perfusion imaging.
Bone Scans: Technetium-99m is used for bone scans to detect abnormalities, fractures, and bone metastases.
Kidney Function Evaluation: Radioisotopes like technetium-99m DTPA assess renal function and detect kidney diseases.
Blood Flow and Brain Studies: Xenon-133 measures cerebral blood flow. Iodine-123 is used for brain imaging.
Radioimmunotherapy: Combines radioisotopes with antibodies to target cancer cells. Yttrium-90 and lutetium-177 are used in this approach.
Therapeutic Radiopharmaceuticals: Radioisotopes deliver targeted radiation to cancer cells. Radium-223 treats bone metastases in prostate cancer.
Research and Drug Development: Radioisotopes aid in studying metabolic pathways, protein binding, and drug interactions.

Storage conditions and precautions of radiopharmaceuticals

Radioactive materials are harmful to human when exposed for longer duration of time. Hence certain precautions are to be taken while working with radioactive materials, detectors, and in handling them. Some of the important handling precautions are given below.

Radioactive materials should never be touched with hand, but should be handled by means of forceps or suitable instruments.
Smoking, eating or drinking should be prohibited in the laboratory, where radioactive materials are present.
Sufficient protective clothing or shields must be used while handling the materials.
Radioactive materials should be kept in a suitable, labelled containers, shielded by lead bricks and preferably in a remote area.
Areas where radioactive materials are stored or used should be monitored (tested for radioactivity).
There should be a proper disposal method for radioactive materials.

Summary

Radiopharmaceuticals are drugs containing radioactive isotopes, serving both diagnostic and therapeutic purposes. They play a pivotal role in modern medicine. Diagnostic imaging relies on radioisotopes for visualizing organs and tissues. Positron emission tomography (PET) uses positron-emitting radioisotopes like fluorine-18. Radiotherapy employs isotopes such as cobalt-60 and iodine-131 for cancer treatment. Cardiovascular studies, bone scans, and kidney function evaluation also benefit from radioisotopes. Additionally, they aid research and drug development. In summary, radiopharmaceuticals enable accurate diagnosis, personalized treatment, and scientific advancements .