RADIOPHARMACEUTICALS

1. RADIOPHARMACEUTICALS

2. The atom is made up of protons and neutrons in the nucleus, with electrons located in discrete energy states around the nucleus. The number of electrons varies from one to over 100 and determines the chemical nature of the atom. The atom has a radius of approximately 10-8cm., with the nucleus having a radius of 10-12cm. • The stability of the atom depends on the neutron to proton ratio (n/z)ratio in the nucleus. • Protons are positively charged with a mass 1800times that of electron. • Neutrons have the same mass like protons but carry no charge.

3. The number of protons of an atom is referred to the atomic number (z). • The number of protons plus the number of neutrons combined the mass number (A). • Isotopes are atoms of various mass number, but have the same atomic number. • Radiation refers to particles or waves coming from the nucleus of the atom through which the atom attempts to attain a more stable configuration. • Radioactivity is the several processes by which atomic nuclei spontaneously decay or disintegrate by one or more discrete energy levels or transitions until a stable state is reached.

6. Types of radioactivity A-Natural radioactivity When the process of decay takes place in a material without the addition of energy B-Artificial radioactivity (man-made radioactivity) In which the radioactivity produced by particle bombardment or electromagnetic radiation: 1-Charged particle reactions : Reactions of these types may be produced with • Protons (11H or P) as 1123Na • Deutrons (12H or d) • Alpha particles (24He or α) • Electrons or beta particles (0-1e or 0-1b)

7. 2-Photon induced reactions Electromagnetic radiations or photons may induce nuclear reactions. The source of electromagnetic energy utilized may be a gamma emitting radionuclide or a high voltage X ray generators 3-Neutron induced reactions One of the most important and also most widely used method for producing artificially radioactive nuclides is the bombardment of a non radioactive target nucleus with a source of thermal neutrons.

8. Radioactive decay • Radioactive species exist in a high exited unstable state, characterized by an excess energy. • These nuclides achieve stability through the process of radioactive decay and the release of large amounts of energy. • In the process of decay there are three parameters which are characteristic for a given radioisotope: -The rate of decay -The type of radiation exhibited in the decay scheme -The energy of theses radiations

9. The equation describe the radioactive decay is a typical first order • N=N0e-λt When λ is the decay constant and the half life is T1/2=0.693/ λ (half life decay) When the activity (A) remaining in a sample after a given time and the intensity of radiation after time, t can be determined. A=A0e-0.693/T1/2 or I=Ioe-0.693/T1/2

10. Mode of radioactive decay 1-Orbital electron capture; • Occurs mainly in the K-shell, in which a nuclear proton is transformed into a neutron by capturing within the nucleus an electron from the K shell and emitting a neutron. This results in the formation of a decay product in which the z number has been reduced by one and there is a vacancy in the K-shell. • After rearrangement of electrons and the emission of specific rays resulting from this rearrangement, the products of such a reaction are the same as if a positron had been emitted. • 2451Cr +-10e 2351V +hν

11. :Measurment of radiation: • Radiation exposure is measured by roentgens. A roentgen (r) is a specific quantity of x or gamma radiation such that the number of ion pairs produced in one cubic centimeter (cc) of air will produced one electrostatic unit of electrical charge (2.58x10-4 coulomb/kg). • The rad is a unit of absorbed radiation dose. The rad represents the amount of energy that has been absorbed per gram of tissue.

12. Different types of radiation may have different relative toxicities. By multiplying the rad dose by a qualifying factor (QF), such as a distribution factor or a specific toxicity factor, the rem dose is obtained (dose equivalent). • Curie(Ci) refers only to the rate of disintegration of radioactive materials. • Curie is 3.7x1010 disintegration per second (dps). • Millicurie (mCi)=3.7x107 dps • Microcurie (mCi)=3.7x104 dps

13. Specific activity is the ratio of any quantity of radioactive material per some unit quantity of total material and has a theoretical unit of 0 to1. It can be measured as μCi/mg, dpm/g. • Specific concentration i.e radioactivity per unit volume, can be utilized in some radiation measurements, e.g. blood volume determination. In blood volume determinations, the administered iv dose in CPM/ml is related to the radioactivity after dilution by the patients. • Carrier free material is a material in which all of the atoms present are radioactive and so it has specific activity of one.

14. Radiation effects on biological systems • The irradiation of biological molecules either in dry state or in aqueous solution leads to a number of chemical and physicochemical changes. These changes results in alteration in enzymatic, hormonal, toxic and immunological function. • The energy absorbed from ionizing radiation can inactivate biological materials in two ways. 1- Direct action which describes the chemical events occuring in the target molecules as a result of radiation energy deposition.

15. These processes include ejection of electrons from atoms in the DNA structure as a result of passage of ionizing photon • This form of damage in the DNA structure was attributed to the radiation induced biological changes in the target molecule in the dry state. 2- Indirect action occurs in aqueous solution in which damages occurs within the biological molecules due to the highly reactive diffusible free radicals produced from the radiolytic products of water.

16. The radiolysis of water as follows: A- The initial physical change lasting only a fraction (10-16) of second in which the energy is deposited in the cell and causing ionization of water H2O ionizing radiation H2O++e- B- The physicochemical stage lasting about 10-16 seconds in which the ions interacts with other water molecules resulting in a number of new products H2O+ dissociation H++OH. H2O+e- H2O- H2O- H. +OH--

17. The existence of oxygen in most biological systems may account for radiosenstivity of cells by forming peroxides • H+ and OH- take a part in the subsequent reactions, the free readicals H. and OH. have an unpaired electron; the hydroxyl radical is a powerful oxidant anh the hydrogen atom is a powerful reductant. Both undergo rapid chemical reaction with organic molecules. C- The chemical stage lasting few seconds, the free radical and oxidizing agent attach the complex molecules and attach themselves to a molecule or cause links in long chain molecules to be broken.

18. The liver appears to be a relatively radioresistant organ due to its large regenerative capacity of its cells and its low oxygen tension of its tissues. • An increase in permeability of capillaries occurred after moderate doses of radiation, this is due to depolymerization of mucopolysaccharides which permits blood to leave the vascular system so haemorrhage was commonly observed following irradiation. • Also irradiation disturbed the enzymatic activity of cabohydrate metabolism.

19. Radiopharmaceuticals are preparations of adequately constant composition, radiochemical, radionucledic purity and uniformity of physiological action for use in nuclear medicine as diagnostic aid or therapeutic agents. A radiopharmaceutical has two components: a radionuclide and a pharmaceutical. A pharmaceutical is chosen on the basis of its preferential localization in a given organ or its participation in the physiologic function of the organ. Then a suitable radionuclide is tagged onto the chosen pharmaceutical such that after administration of the radiopharmaceutical, radiations emitted from it are detected by a radiation detector

20. Radiopharmaceuticals are preparations of adequately constant composition, radiochemical, radionucledic purity and uniformity of physiological action for use in nuclear medicine as diagnostic aid or therapeutic agents.

21. The practice of radiopharmacy • A-Formulation and dispensing: The following factors need to be considered before, during and after the preparation of a new radiopharmaceutical: 1-Compatibility: When a labeled compound is to be prepared, the first criterion to be considered is whether the label can be incorporated into the molecule to be labeled. This may be assessed from the knowledge of the chemical properties of the two partners.

22. For example, 99mTc ion can form coordinate covalent bonds with ligands containing O, N or S atoms with lone pair of electrons that can be donated to form coordinate covalent bonds. When 99mTc ion and ligand like DTPA are mixed under appropriate physicochemical conditions, 99mTc-DTPA is formed and remains stable for a long time. If, however, 99mTc ion is added to benzene or similar compounds, it would not label them. Iodine primarily binds to the tyrosyl or hestidyl group of the protein. Mercury radionuclides bind to the sulfhydryl group of the protein.

23. 2-Charge of the molecule: The charge on a radiopharmaceutical determines its solubility in various solvents. The greater the charge, the higher the solubility in aqueous solution. Non polar molecules tend to be more soluble in organic solvents and lipids. 3-Size of molecule: The molecular size of a radiopharmaceutical is an important determinant in its absorption in the biologic system. Larger molecules (M.W  60000) are not filtered by the glomeruli in the kidney. This information should give some clue about the range of molecular weight of the desired radiopharmaceutical that should be chosen for a given study.

24. 4-Stoichiometry: In preparing a new radiopharmaceutical, one needs to know the amount of each component to be added. This is particularly important in tracer level chemistry and in 99mTc chemistry. The concentration of 99mTc in the 99mTc-eluate is approximately 10-9 M. Although for reduction of this trace amount of 99mTc requires only an equivalent amount of Sn2+ is needed, 1000 to one million times more of the latter is added to the preparation in order to ensure complete reduction. Similarly, enough chelating agent is also added in order to consume all the reduced 99mTc.

25. 5-Protein binding: Almost all drugs, radioactive or not, bind to plasma proteins to variable degrees. The primary candidate for this type of binding is albumin, although many compounds specifically bind to globulin and other proteins as well. Protein binding is greatly influenced by a number of factors, such as the charge on the radiopharmaceutical molecule, the pH, the nature of protein and the concentration of anions in plasma. At a lower pH, plasma proteins become more positively charged, and therefore anionic drugs bind firmly to them.

26. The nature of a protein, particularly its content of hydroxyl, carboxyl and amino groups and their configuration in the protein structure determines the extent and strength of its binding to the radiopharmaceutical. Metal chelates can exchange the metal ions with proteins because of the stronger affinity for the metal. Such process is called “transchelation” and leads to in-vivo breakdown of the chelate. Protein binding affects the tissue distribution and plasma clearance of a radiopharmaceutical and its uptake by the organ of interest. Therefore, one should determine the extent of protein binding of any new radiopharmaceutical before its clinical use.

27. 6-Solubility: For injection, the radiopharmaceutical should be in aqueous solution at a pH compatible with blood pH 7.4. The ionic strength and osmolarity of the agent should also be appropriate for blood. In many cases, lipid solubility of a radiopharmaceutical is a determining factor in its localization in an organ. The cell membrane is primarily composed of phospholipids, and unless the radiopharmaceutical is lipid soluble, it will hardly diffuse through the cell membrane. The higher the lipid solubility of a radiopharmaceutical, the greater the diffusion through the cell membrane and hence the greater its localization in the organ.

28. Protein binding reduce the lipid solubility of a radiopharmaceutical. Ionized drugs are less lipid soluble, whereas non-polar drugs are highly soluble in lipids and hence easily diffuse through cell membrane. Lipid solubility and protein binding of any drugs play a key role in its in-vivo distribution and localization 7-Stability: The stability of a labeled compound is one of the major problems in labeling chemistry. It must be stable both in-vitro and in-vivo. Temperature, pH and light affect the stability of many compounds and the optimal range of these physicochemical conditions must be established for the preparation and storage of labeled compounds. • 1.7.2.2 Short effective half-life: • A radionuclide decays with a definite physical half-life. The physical half-life is independent of any physicochemical condition and is characteristic for a given radionuclide. Radiopharmaceuticals administered to humans disappear from the biologic system through fecal or urinary excretion, perspiration or other mechanisms. This biologic disappearance of a radiopharmaceutical follows an exponential law similar to that of radionuclide decay. Thus, every radiopharmaceutical has a biological half-life.

29. In-vivo breakdown of a radiopharmaceutical results in an undesirable biodistribution of radioactivity. 8-Biodistribution: Studying of the biodistribution of a radiopharmaceutical is essential in establishing its efficacy and usefulness. The rate of localization of a radiopharmaceutical in an organ is related to its rate of plasma clearance after administration.

30. B-Product development: • The extent to which a radiopharmacist is required to function in this area depends on the complexity of the nuclear medicine department that utilizes his services. • If a significant amount of clinical research is carried out, product development can become an exceedingly important aspect of the practice of radiopharmacy. C-Quality control: The radiopharmaceutical quality control include I-Pharmaceutical consideration: a-Inspection of label amount

31. Product identity • Manufaturer-name&address • Product list&lot number • Total radioactivity (time, date) • Concentration (time, date) • Specific activity • Diluent • Additive • Expiration date • Storage conditions

32. B-Appearance • Color • Clarity • Particle size C-pH determination of liquid formulation D- Biological testing or certification • A pyrogenicity and sterility (parenterales) • Acute toxicity and safety • Chronic toxicity • Efficacy

33. II-Radiation consideration: Assuming product contains mixed beta-gamma or pure gamma emitters A- Radionuclidic assay • Total activity in containers • Activity per unit volume • Dose calibration B- Radiation purity • Radio chromatography • Electrophoresis C- Removable contamination • Smear of surface of container

35. Groups of 99mTc-radiopharmaceuticals: 1-Pertechnetate ion (99mTcO4-): Technetium-99m eluted directly from 99Mo/99mTc generator as the 99mTcO4- ion, which was first evaluated as possible biological tracer. When 99mTcO4- administrated intravenously in human patient, it haswide spread use for brain tumor localization and for thyroid imaging. 2-99mTc-Labeled colloids and particulates: 99mTc-sulfur colloid is prepared by heating mixture of 99mTcO4- and sodium thiosulphate in acidic medium for 5 to 10 min in boiling water bath. Gelatin is added before the reaction with the acid in order to stabilize sulfur in colloid state. It is used for scintigraphy of the recticuloendothelial system.

36. 3 -99mTc-Chelates for skeletal imaging: These 99mTc-chelates are used for skeleton scintigraphy such as MDP, HEDP, HMDP, pyrophosphates and polyphosphates. Diphosphonates chelating agents are analogue to pyrophosphate whose P–O–P structure is replaced by P–C–P which is more stable against enzymatic decomposition by phosphatase enzyme. The organic phosphonates are more stable than the inorganic phosphates against in-vivo metabolism.

37. 4-99mTc-Chelates for renal imaging: Several 99mTc-chelates, like a variety of organic acids and bases are filtered by the glomerulus. A few of them may be partially secreted by the proximal tubular cells and may undergo partial tubular reabsorption passively depending on their Pka,s, lipid solubility, and pH of the tubular fluid. The tubular cells retain a variety of metal ions by chelation with thiol groups present in these proteins. Numerous 99mTc-complexes are frequently used for kidney function study such as 99mTc iron-ascorbate, diethylene triamine pentaacetic acid

38. 5- 99mTc-Chelates for myocardial imaging: The cationic complexes of 99mTc (III) with the neutral ligands of arsine and phosphine like the monovalent alkali metal ions localize in the muscle cells. The cationic technetium-99m complexes, alkyl isonitriles complexes, where the alkyl groups are methyl, ethyl, tertairy butyl or methoxy isobutyl were synthesized.

39. 6- 99mTc-Complexes for brain imaging: The principle of brain imaging is governed by a mechanism called blood brain barrier (BBB), which excludes many substances from entering the brain from the blood. The BBB is probably a function mixture of anatomic, physiologic, and metabolic phenomena, and which of these are effective in a particular instance depends on the physicochemical properties of the substance in question. Recently, two groups of ligands have been studied extensively for their ability to form neutral lipid soluble complexes with reduced technetium capable of penetrating the blood brain barrier

40. The first group comprises diaminodioxime derivatives, while the second comprises diaminodithiol derivatives. Several derivatives of propylene amineoxime (PnAO) were synthesized by different methyl substitutions on the amineoxime backbone. The most important one of these derivatives is d,l–hexamethyl propylene amine oxime (d,l-HMPAO). 99mTc-d,l-HMPAO is neutral lipid soluble complex used for measurement of cerebral perfusion of brain by SPECT technique (36). The 99mTc complex of L,L-ethylcysteinate dimer (ECD) demonstrated cerebral uptake and longer retention time in brain.

41. 7- 99mTc-Chelates for hepatobillary imaging: The derivatives of iminodiacetates (IDA) are excellent chelating agents for 99mTc. The first complex involved is 2,6–dimethylacetanilido-imino-diacetate .99mTc-IDA derivatives are prepared by the direct reduction of pertechnetate with Sn(II) in presence of the ligand (IDA) where a 99mTc-IDA complex is formed. A variety of 99mTc-chelate of IDA derivatives were evaluated clinically, the 3-bromo-2,4,6-trimethyl, iodo and tertiary butyl derivatives of IDA were found to be excellent ligands.

42. 8-99mTc-Complexes for lung imaging: Lung perfusion imaging is based on the trapping of large particles in the capillary bed of the lungs. Particles larger than 10 mm are lodged in the capillary in the first pass of circulation through the pulmonary artery following intravenous administration. The most widely used 99mTc-labeled particles for lung study are: • 1- Microspheres of denatured human serum albumin. • 2- Macroaggregated albumin (MAA). • 3- 99mTc-labeled Aerosol.

43. 9- 99mTc-Complexes of proteins: Several proteins (human serum albumin, neoga–lactoalbumin, monoclonal and polyclonal antibodies) have been labeled with 99mTc radionuclide. The labeling was accomplished by direct chelation of macromolecules (which contain large number of binding sites) with reduced 99mTc or by indirect conjugation of 99mTc-complex (precomplexing agent method) to protein via bifunctional chelating agent .

44. Good radiopharmaceutical requirements: Since radiopharmaceuticals are administered to humans, they should possess some important characteristics. The ideal characteristics for diagnostic radiopharmaceuticals are:  1-Ease of availability: The radiopharmaceutical should be easily produced, inexpensive andreadily available in any nuclear medicine facility. Complicated methods of production of radionuclide or labeled compounds increase the cost of the radiopharmaceuticals.

45. 2-Short effective half-life: • A radionuclide decays with a definite physical half-life. The physical half-life is independent of any physicochemical condition and is characteristic for a given radionuclide. Radiopharmaceuticals administered to humans disappear from the biologic system through fecal or urinary excretion, perspiration or other mechanisms. This biologic disappearance of a radiopharmaceutical follows an exponential law similar to that of radionuclide decay. Thus, every radiopharmaceutical has a biological half-life.

46. 3-No particle emission: • Radionuclides decaying by  or - particle emission should not be used in labeling of radiopharmaceuticals. These particles cause more radiation damage to the tissue than -rays do. Although -ray emission is preferable, many - emitting radionuclides, such as 131I-iodinated compounds, are often used for clinical therapeutic studies. However, -emitters should never be used for in-vivo clinical studies because they give a high radiation dose to the patient.

47. 4-Decay by electron capture or isomeric transition: • Because radionuclides emitting particles are less desirable, the radionuclides used should decay by electron capture or isomeric transition without any internal conversion. Whatever the mode of decay, the radionuclide must emit -radiation with energy between 30 and 300 KeV. Below 30 KeV, -rays are absorbed by tissue and are not detected by the NaI (TI) detector. Above 300 KeV, effective collimator of -rays can not be achieved with lead or denser metal.

48. 5-High target to non- target activity ratio: • For any diagnostic study, it is desirable that the radiopharmaceutical must be localized preferentially in the organ under study since the activity from non-target areas can obscure the structural details of the picture of the target organ. Therefore, the target to non-target ratio should be high.

49. Preparation of injectable radiopharmaceuticals: Only the purest grade of chemicals or those of stated purity should be used in the preparation of radiopharmaceuticals and dilutions. Solutions of dilute acids and alkalis used for pH adjustment or in the preparation should be prepared from concentrated acid or from 1:1 by weight sodium hydroxide solution, in sterile pyrogen-free water. Dilute solutions may be sterilized by filtration through a 0.22 m pore size membrane filter, since heat sterilization may cause the formation of silica flakes from the glass or breakdown of the active ingredients.

50. Preparation of injectable radiopharmaceuticals Ligands: • In general, complex formation occurs for compounds having functional groups such as hydroxyl, carboxyl, amino, mercapto and phosphonate groups. Bidentate and multidentate ligands are most commonly used. High oxidation states of technetium prefers ligands that are able to compensate the very high positive charge of the central atom. In the oxidation state of +5, technetium is stabilized as oxotechnetium or nitrotechnetium, where the lower oxidation states prefer ligands containing soft donor atoms such as sulphur, phosphorus and arsenic ones.