CONTENTS

1. Basic principles of the targeted delivery of radionuclides

2. Targeted radiopharmaceuticals

2.1. Diagnostic and therapeutic radionuclides

2.2. Targeting Mechanisms

3. Vector carrier molecules

3.1. Antibodies and their derivatives

3.2. Antibody analogues

3.3 Peptides

3.4. Microspheres and nanoparticles

Conclusions

INTRODUCTION

The history of the development of nuclear-physics methods used in medical practice goes back more than a century.

On November 8, 1895, Wilhelm Conrad Roentgen discovered X-rays, which became the starting point in the formation of areas of radiation diagnostics and therapy.

In Russia, the first experimental work on the study of X-rays was carried out already at the beginning of 1896. Russian scientists not only repeated the experiments of W. C. Roentgen, but also became the founders of the clinical application of X-ray radiation (N.G. Egorov, A.S. Popov, and R.A. Popova, S.Ya. Tereshina, A.K. Yanovsky, S.P. Fedorov, M.I. Nemenov, and I.I. Borgman) [1].

Another great event that laid the foundation for the development of nuclear medicine was the discovery of spontaneous radioactivity in early 1896 by A. Becquerel. Soon, Pierre and Marie Curie (Skłodowska-Curie) began studying the spontaneous emission of matter (nuclei), and radium was discovered in 1898 [2].

In 1901, French physicists A. Danlos and E. Bloch were the first to use radium for the treatment of tuberculosis of the skin. Danlos also studied the possibility of using radium for the treatment of skin manifestations of systemic lupus erythematosus, and in 1903 Bell proposed the use of radium for the treatment of tumors [3].

In 1908, radium was first used for the treatment of uterine cancer, and in 1909, for the intracavitary treatment of prostate cancer [4], which served as the basis for the emergence of brachytherapy, a type of radiotherapy in which a radiation source is introduced into the affected organ.

Increasing interest in the use of sources of ionizing radiation in medical practice has determined the need for a more in-depth study of the mechanisms of radiation effects on various organs and tissues. In 1903, the distinguished Russian scientist E.S. London was the first to show that long-term radium exposure can have a lethal effect on mice [5, 6]. During the period 1904–1909 he continued research into the medical and biological effects of radium radiation associated with the influence of radium (radon) emanation on living organisms. This is how the world’s first monograph on radiobiology appeared: Radium in Biology and Medicine [7]. Subsequently London not only established the patterns of differentiated radiosensitivity and radioresistance of different organs and tissues, but also, foreseeing the significant role and prospects of nuclear medicine, convinced Emperor Nicholas II to allocate funds for the development of nuclear science in Russia, which, according to the scientist, was to determine the future power and the power of nations [8].

In 1913, D. de Hevesy proposed the method of isotope indicators, and in 1925, H. Blumgart performed the first radiodiagnostic procedure involving the intravenous injection of a radioactive mixture of 226Ra decay products to a patient. This is how the new directions of radionuclide diagnostics and therapy emerged.

The further development of nuclear medicine required expansion the range of available radionuclides. It became obvious that it was necessary to create special installations to produce artificial radioisotopes, the fundamental possibility of obtaining which was established by E. Rutherford in 1919.

So, in 1929 E. Lawrence invented a 27-inch cyclotron (a resonant cyclic accelerator of protons and deuterons with an energy of 4.8 MeV; in 1937, on its basis, he created a 37-inch cyclotron with an ion-acceleration energy of up to 8 MeV, and in 1939, a 60-inch (16 MeV) cyclotron, which became the main tool for obtaining radionuclides [9].

His brother J. Lawrence, M.D., during the same period demonstrated the value of cyclotrons for the production of isotopes for medical purposes. Since 1936, under his leadership, patients with leukemia and polycythemia have been treated using radioactive 32P produced on a cyclotron. These were the first therapeutic applications of artificially produced radioisotopes in humans. Thus, John Lawrence became the “father of nuclear medicine.”

In Russia, there was also active development of accelerator technologies and, in parallel, in 1937, the first cyclotron in Europe with a diameter of 1 m and an energy of 3.2 MeV was launched at the Radium Institute (Leningrad).

A powerful impetus for the further development of nuclear-physics technologies used in modern medicine was the first atomic project [10].

The creation, under the scientific guidance of I.V. Kurchatov, of the first cyclotrons and reactors based on Laboratory No. 2 of the USSR Academy of Sciences, later renamed the National Research Center “Kurchatov Institute,” led to the emergence of a huge network of accelerator and reactor centers throughout the country, and the developed methods for enriching and separating uranium isotopes became the basis for modern technologies for obtaining radionuclides for nuclear medicine.

In 1944, the physical launch of the M-1 cyclotron was carried out, which made it possible to produce the first proton beam in the USSR and obtain the first samples of plutonium, laying the foundation for radiochemistry. Soon, in 1946, the largest cyclotron was launched at Leningrad Physical—Technical Institute (LPTI).

The appearance of the first cyclotrons (in 1937, 1944, and 1946) became a powerful impetus for the development of accelerator technologies in Russia: in 1947, the U-150 accelerator was created at the Kurchatov Institute, followed by the specialized synchrotron KISS-Kurchatov in 1999; as well as a whole a number of accelerators: proton accelerators U-7 and U-70 at the Institute of Nuclear and Experimental Physics, National Research Center “Kurchatov Institute,” (Laboratory no. 3); U-120 at the Ioffe Physical—Technical Institute; the synchrocyclotron and NICA at the Joint Institute for Nuclear Research, Dubna (Hydrotechnical Laboratory); proton synchrotron U-70 at the Institute for High-Energy Physics, National Research Center “Kurchatov Institute”; STs-1000 and Ts-80 at the Konstantinov Petersburg Nuclear Physics Institute, National Research Center “Kurchatov Institute” (branch of the Ioffe Physical–Technical Institute); VEP-1 at the Budker Institute of Nuclear Physics (Laboratory of New Acceleration Methods, Institute of Atomic Energy); U-150M at the Nuclear Physics Institute of the Republic of Kazakhstan, etc.

On December 25, 1946, under the scientific guidance of Kurchatov, F-1 which was the first reactor in the USSR and Europe was launched; in 1948, the Annushka reactor was launched, which laid the foundation for the creation of a huge branch, the so-called “Kurchatov Tree,” of new nuclear reactors differing in design features and operating principles [11]. Today, the created installations represent the basis not only of modern power engineering, military and civilian icebreaker fleets, and solve problems of the state’s defense capability, but also contribute to the active development of technologies for nuclear medicine and radiation therapy and ensure Russia’s presence in the top five leaders in the world market of isotope products.

It was in the post-war period, in connection with new tasks for the use of atomic energy for peaceful purposes, that the main directions became the study of the pathogenesis, clinical picture, prevention and treatment of radiation sickness, and the development of methods for the use of radioactive isotopes and other types of ionizing radiation for diagnosis and treatment. To study the effect of radiation on humans and develop protective equipment in 1946, on the initiative of Kurchatov, a radiation laboratory was created in the USSR Academy of Sciences, later renamed the Institute of Biophysics, which was entrusted with the study of the biological effects of radiation. This direction was continued in the radiobiological department of the Konstantinov Petersburg Nuclear Physics Institute, National Research Center “Kurchatov Institute,” then still a branch of the Physical–Technical Institute of the USSR Academy of Sciences, where in 1964 a laboratory of radiation genetics was opened, and later of general radiobiology, molecular biology, and organic synthesis. Particular attention was paid to ensuring biological experiments on the biological channel of the VVR-M reactor, launched at the same institute in 1959 [12]. It was precisely this interdisciplinary approach and coordinated work of scientists and engineers that allowed the head of the molecular biology department S.E. Bresler to create a new direction and specialty “biophysics.” Today, on the basis of this department, the center for preclinical and clinical research of the Kurchatov Institute has been created and rather successfully operates. It has become the first specialized platform in the Russian Federation for the preclinical research of radiopharmaceutical drugs (RPs).

The active development of molecular biology in the mid-20th century led to the formation of a completely new concept of the “magic bullet,” proposed by the German scientist and medical practitioner Paul Ehrlich, according to which target receptors located on the outside of the cell of a pathogenic organism and absent in a patient’s body are able to bind to specific chemicals. Subsequently, this theory, initially used to create antibacterial drugs, became the basis for the emergence of a new generation of targeted antitumor drugs capable of the strictly specific destruction of tumor cells [13].

As a result of the combination of achievements in the field of radiobiology and molecular oncology at the end of the 20th century, a separate direction of targeted radionuclide therapy was formed using targeted radiopharmaceuticals consisting of a pharmaceutical substance (PS), providing delivery to the target receptor of an open source of ionizing radiation, i.e., a radionuclide, which affects a malignant formation (tumor cells). Until the 1980s Russia kept pace with the world’s leading countries in the field of introducing nuclear-physics methods into medicine, but in the 1990s and early 2000s there was a sharp decline. The Kurchatov Institute then realized the criticality of the situation: in 2007, E. P. Velikhov and M. V. Kovalchuk turned to the country’s leadership with a project to revive nuclear medicine, thanks to which areas related to the use of nuclear-physics methods in medicine have found a second wind and today are being actively developing within the walls of the Kurchatov Institute within the framework of a research and technological complex of convergent nano-, bio-, information, cognitive, and social-humanitarian natural sciences and technologies (NBICS-nt) that has no direct analogs in the world [10].

Modern trends in the development of nuclear-physics methods are primarily aimed at:

(i) increasing the accuracy and reliability of diagnostic studies;

(ii) minimizing the impact of ionizing radiation on surrounding healthy tissue;

(iii) improvement of technologies for the treatment of radioresistant malignant neoplasms, distant metastases, and disseminated tumors;

(iv) increasing the effectiveness of treatment through the early diagnosis of diseases and increasing the accuracy of therapeutic effects, as well as the use of combined treatment methods;

(v) ensuring the monitoring of therapeutic effects through the use of theranostic radiopharmaceuticals.

Today, conventional therapy technologies have made it possible to improve the quality of dose distributions and reduce the volume of healthy tissue irradiated (intensity-modulated radiation therapy (IMRT) and image-guided radiation therapy (IGRT) technologies), and to increase the accuracy of the dose-anatomical planning system using fusion technologies of computer tomography (CT), magnetic resonance imaging (MRI), and positron-emission tomography (PET).

To minimize radiation exposure to healthy tissue, methods of hadron-beam therapy have undergone active development: proton (PBT), using accelerated beams of protons, and ion (IBT), using heavy carbon ions. The physical advantage of these methods lies in the ability to most accurately localize radiation exposure in the area of a given volume (provide conformal irradiation) due to implementation of the Bragg-peak effect [14]. In addition, since carbon ions with high linear energy transfer (LET) cause complex DNA damage (dense ionizing radiation), IBT is an effective method for action against radioresistant tumors.

In order to increase the effectiveness of the treatment of distant metastases and disseminated tumors with minor side effects and minimal damage to normal tissue, a method of radionuclide therapy based on the use of targeted RPs is actively used. The delivery mechanisms of such drugs are based on the specific recognition of molecular abnormalities characteristic of neoplasms [15, 16].

In recent decades, it has become obvious that the microenvironment of solid formations plays an important role in the occurrence, development, metastasis of cancer tumors, as well as their resistance to drug exposure. Thus, stromal cells and other components of the extracellular matrix, as well as changes in metabolic processes (hypoxia, acidity, increased metabolism) can contain valuable prognostic and pathophysiological information and are considered as an informative marker and/or therapeutic target for targeted radionuclide diagnostics and therapy [17–19]. Taking into account the active introduction of personalized medicine approaches into everyday clinical practice, the field of theranostics, which involves the use of the same vector molecule labeled with different radioisotopes for diagnostic and treatment purposes, is gaining increasing interest [20]. This approach has undeniable advantages in planning and optimizing the management of each individual patient, and in increasing the effectiveness of therapy and minimizing the dose load [21].

Among the combined methods of treating oncological diseases, special attention is paid to the combination of nuclear-physics methods and immunotherapy [22]. Compared with external-beam radiation therapy, radionuclide therapy shows better results when combined with immunotherapy for patients with secondary (metastatic) lesions. However, many questions still remain related to the mechanisms of the antitumor immune response against the background of ionizing radiation and optimal approaches to the clinical use of these combinations [23].

To quickly solve these problems, of course, government support is required, which will allow us to move to a qualitatively new level of providing high-tech assistance to the Russian people.

A striking example is the Federal Scientific and Technical Program for the Development of Synchrotron and Neutron Research and Research Infrastructure for 2019–2027, approved by Decree of the Government of the Russian Federation, March 16, 2020, No. 287. Within its framework, it is envisaged to create a scientific and educational medical center for nuclear medicine on the basis of the National Research Center “Kurchatov Institute” (SEMC NM), which includes modernized complexes of ion (carbon) and proton radiation therapy, as well as a radioisotope complex for the production of a wide range of medical radionuclides for the creation of radiopharmaceuticals. The activities of the SEMC NM will ensure the development, accelerated implementation, and rapid dissemination of advanced technical means and technologies of medical radiology based on Russian equipment, including for the production of promising radionuclides and the creation of corresponding RPs based on them [24].

1 BASIC PRINCIPLES OF THE TARGETED DELIVERY OF RADIONUCLIDES

Modern methods of radionuclide diagnostics and therapy, based on the introduction into the patient’s body of appropriate diagnostic, therapeutic, and theranostic radiopharmaceuticals, are widely used not only in oncology [25] and cardiology [26], but also in endocrinology [27], psychiatry, neurology [28], osteology [29], pediatrics [30], and a number of other areas of medicine.

A distinctive feature of RPs from other drugs is the presence in their composition of a source of ionizing radiation, i.e., a radionuclide, which determines the nature and degree of impact on a given area of interest. During the process of radioactive decay, emitted particles or γ quanta are either recorded by an external detector, which subsequently makes it possible to reconstruct the image and visualize the area of interest under study (pathological changes), or to influence pathological foci in organs and tissues for therapeutic purposes.

The possibility of using a radionuclide for diagnostic or therapeutic purposes is determined by:

(i) its nuclear physical characteristics (half-life, type of radiation, LET, relative biological effectiveness (RBE) etc.) [31];

(ii) chemical properties (for example, the possibility of obtaining stable biomolecules containing a radionuclide);

(iii) technical and economic feasibility (possibility of scaling technologies, economic feasibility of its production, etc.).

The basic principle of targeted nuclear medicine is based on ensuring the targeted delivery of a radionuclide to the area of interest and minimizing the impact on healthy organs and tissues.

However, few radioisotopes, when incorporated into the human body, are able to accumulate in certain tissues and cells. The classic example is 131I, actively absorbed by the follicular cells of the thyroid gland. Since the 1940s it is actively used in the treatment of thyroid cancer and thyrotoxicosis due to its ability to interact with the transmembrane protein (glycoprotein), i.e., the sodium-iodine symporter, which allows the transfer of iodine ions through the basement membrane [32, 33].

Another natural process leading to the accumulation of 223Ra or 89Sr in an area of increased bone turnover is based on their physical and chemical similarity to calcium. Radium-223 dichloride and strontium-89 chloride can be incorporated into the crystal structure of the bone mineral hydroxyapatite instead of calcium, which allows them to be used for the treatment of osteoblastic (sclerotic) metastases [34], most often found in breast cancer, prostate cancer, and small cell lung cancer.

The vast majority of radionuclides cannot be selectively captured by certain types of tissues and cells; therefore, in order to ensure targeted delivery of the radionuclide to the target, special PS are used, i.e., carrier molecules responsible for the biodistribution of RPs [15].

The main condition for the successful use of labeled compounds in oncology is their high sensitivity and specificity for malignant neoplasms. Such targeted delivery of RPs can be carried out through active and passive targeting mechanisms.

The strategy of the passive delivery of radionuclides and corresponding radiopharmaceuticals is carried out through the accumulation of carrier molecules in a certain place due to their inherent pathophysiological, physical—chemical, or pharmacological factors. This strategy is based on the effect of an abnormal increase in the permeability of the vascular network, discovered in 1986, with subsequent retention in tumor tissue: the enhanced permeability and retention (EPR) effect [35]. Due to excessive vascular proliferation caused by the abnormal need of a solid tumor for oxygen and nutrition, large pores (fenestrations) are formed between endothelial cells (from 380–780 nm to 1.2 μm depending on the type of tumor) (Fig. 1), which leads to the increased permeability of tumor capillaries compared to capillaries in normal tissues and, in combination with impaired lymphatic drainage, the accumulation of macromolecular compounds (usually above 40 kDa) in the vascularized area of the tumor. For passive targeting, nanoparticles of various natures are used: inorganic, including noble metals, magnetic metals, quantum dots and nonmetals; organic, consisting of polymers and lipids (liposomes, albumin nanoparticles, dendrimers, polymer micelles, polymer nanoparticles); carbon nanoparticles [36–40].

Fig. 1.
figure 1

Targeted delivery of radionuclides and radiopharmaceuticals.

Full size image

The active delivery mechanism is designed to transport toxins [40, 43], drugs [44], diagnostic reagents, and cytotoxic radionuclides [45–47] into the tumor cell. It involves the use of targeted carrier molecules capable of specifically interacting with molecular targets, which are usually located on the surface of tumor cells (Fig. 1).

Having a high affinity for binding to a specific type of cell or tissue in the body, targeted carrier molecules can reduce the risk of irradiation of healthy tissues and reduce the required dose of administered radiopharmaceuticals, thereby increasing the accuracy and efficiency of treatment.

Targeting ligands such as antibodies, their fragments and derivatives, scaffold proteins, peptides and small molecules are used as target carrier molecules [48, 49]. They can also be used for the targeted delivery of radiolabeled nanoparticles (a combination of active and passive targeting strategies). For the targeted delivery of such radiopharmaceuticals, targets located not only on the tumor cells themselves, but also on normal cells of the tumor microenvironment, for example, endothelial cells of blood vessels, can be selected, followed by the release of encapsulated agents [50].

In recent years, a promising approach has emerged based on the delivery of a radionuclide to a specific organelle of the target cell. This mechanism of subcellular targeting is the most promising when α emitters and Auger-electron emitters are used; however, it requires improved methods for determining the localization and visualization of RPs, as well as a more detailed study of biochemical and physical–chemical processes and assessment of the risks of accumulation in healthy tissues [51, 52].

2 TARGETED RADIOPHARMACEUTICAL MEDICINES

2.1 Diagnostic and Therapeutic Radionuclides

Depending on the energy and the predominant type of radiation, radionuclides used in medical practice are usually divided into diagnostic and therapeutic types.

For diagnostic purposes They mainly use radionuclides with short half-lives (minutes, hours, tens of hours), emitting (Table 1):

Table 1. Basic parameters and properties of various types of radiation used in radionuclide therapy
Full size table

(i) γ quanta (photons) with fairly low energies (50–400 keV), released as a result of the isomeric transition (99mTc) or K capture (111In, 123I, etc.);

(ii) positrons, which, when interacting with electrons, annihilate, releasing two γ quanta with an energy of 511 keV (18F, 82Rb, 124I, etc.).

Most of them (except 99mTc) are neutron-deficient nuclei and can be produced in charged-particle accelerators.

Single γ quanta are recorded using a γ chamber. To obtain two-dimensional images of the distribution of γ-emitting radionuclides in a patient’s body, planar γ scintigraphy is used, and to create three-dimensional images, single-photon emission computed tomography (SPECT) (Table 2).

Table 2. Radionuclides for SPECT and planar γ scintigraphy
Full size table

To clearly highlight the area in which the radiation source is located, special collimators of various geometric shapes and sizes are used. The most commonly used are plane-parallel collimators. The channels in them are made in the form of holes located on the same line.

The key parameter for γ-emitting radionuclides is the photon energy. Based on it, the thickness of the partitions (septa) between the lines and their number are designed. Low-energy γ quanta (~70 keV) are strongly absorbed in tissue, which leads to significant distortions in images. Low-energy collimators have a thin septum and a large number of holes (up to several thousand) [53].

The greater the thickness of the collimator, the lower the efficiency of γ-ray recording, but the better the spatial resolution. However, at high energies the required collimator thickness becomes unreasonably large. Therefore, as a rule, γ-emitting radionuclides with energies of >400 keV are not used for diagnostic purposes.

Until now, the most popular radionuclide used in nuclear medicine remains 99mTc [54]. With its participation, about 40 million procedures per year are performed worldwide, which is ~80% of all radionuclide studies and 85% of diagnostic scans [55]. This radionuclide is formed from 99Mo (half-life of 66 h), on the basis of which the 99mTc generator is made, which makes technetium accessible and fairly cheap to produce. Its half-life (6 h) allows imaging on the day of injection, but it is possible to image 24 h after nuclide administration. Modern SPECT scanners are optimized for image acquisition using 99mTc.

To record annihilation radiation from positron-emitting radionuclides, the PET method is used, which is based on a coincidence pattern. The simultaneous recording of γ quanta emitted during positron annihilation helps determine the line along which annihilation occurred. Processing the information obtained from multiple recording makes it possible to reconstruct the distribution of activity.

The spatial resolution in PET is determined by the range of positrons in the tissue to the point of annihilation and some other parameters [56]. The acceptable resolution is ~5 mm. The distance to the point of annihilation (collision) is a variable value for different radionuclides, this explains the different resolution of the radiogram for different RPs.

The PET method is traditionally associated with the application of short-lived “biogenic” radionuclides (Table 3), the use of which makes it possible to label organic biologically active molecules without changing their biological and chemical properties. Thus, PET is a unique tool for studying in vivo biochemical processes and their disorders associated with various diseases.

Table 3. Short-lived radionuclides for PET
Full size table

The growing interest and availability of nonstandard positron-emitting radionuclides (e.g., 52Mn, 86Y, 89Zr) have increased the relevance of the choice of radionuclides in the development and optimization of new protocols for PET studies using both hardware and software methods. In [57], correction algorithms were proposed at the stage of image reconstruction using the Monte Carlo method to improve the clarity of PET images.

In general, radionuclides used to create diagnostic RPs are subject to a number of recommended requirements [58]:

(i) absence or minimization of the radiation yield of β– particles that do not carry diagnostic information, but increase the dose load on normal organs and tissues;

(ii) minimum average range of positrons in biological tissues, i.e., minimizing the distance from the emission point of β+ particles to the point of their annihilation and, as a result, the higher quality of PET imaging;

(iii) the maximum possible radiation yield of β+ particles in radionuclides used for PET, or the presence of a single line of γ radiation with an energy of 100–200 keV for radionuclides used in SPECT;

(iv) absence or minimal yield of byproducts of γ radiation arising from the decay of some positron-emitting radionuclides and leading to an increase in the number of false matches in PET;

(v) the half-life should be equal to several hours: a half-life that is too long will increase radiation exposure of the patient due to excessively prolonged internal exposure after research, and too short will lead to the need to introduce a correction for decay during research;

(vi) the specific effective dose of diagnostic radiation in units of mSv/MBq (i.e., per 1 MBq of activity of RPs introduced into the patient’s body) should be the minimum possible;

(vii) the radionuclide production technology must be accessible for its use in mass production and economically feasible.

According to global statistics [59], in developed countries, ~2% of the population annually undergoes research using nuclear-medicine methods, of which only 10% accounts for radionuclide therapy. Despite this, interest in radionuclide therapy is growing every year. The development of nuclear technology has made available a fairly wide range of potential therapeutic radionuclides with various physical and radiobiological properties.

The degree of cellular damage from exposure to ionizing radiation depends on a number of factors, including the absorbed radiation dose, dose rate, area parameters (geometric characteristics, heterogeneity, etc.), type of irradiation, path length, and LET. Radionuclides that emit α or β particles, or Auger electrons [60] are used for therapy. The dose rate and tissue dose distribution are aspects of radiation exposure that can be varied experimentally or during treatment. At the same time, the type of irradiation, path length and LET directly characterize the radionuclide itself (Table 1). Radiation with high LET determines a greater concentration of energy over a shorter distance when passing through tissue compared to radiation with low LET. High LET leads to intense damage to macromolecules and cell death through numerous mechanisms, such as the radiolysis of water and lipid peroxidation, the ionization of macromolecules, the formation of single- and double-strand breaks, etc.

Today, radiopharmaceuticals are used for the synthesis of targeted therapeutic drugs (Tables 4–6):

Table 4. Positron-emitting nuclides for labeling targeting proteins
Full size table
Table 5. Physical properties of some β-emitting nuclides potentially suitable for targeted radionuclide therapy
Full size table
Table 6. α-Emitting nuclides potentially suitable for targeted radionuclide therapy
Full size table

(i) α emitters with high LET (~80 keV/µm) and a short particle path length (50–90 µm), for example 211At and 212Bi;

(ii) β emitters with a relatively short particle path length (520 µm), for example 33P, 121Sn, 177Lu, 191Os, and 199Au;

(iii) β emitters with an average particle path length (≥200 µm with an average value of ~1 mm), for example 47Sс, 67Cu, 77As, 109Pd, 111Ag, 131I, 143Pr, 161Tb, and 188Re;

(iv) β emitters with a relatively large particle range length (on average >1 mm), for example 32P, 90Y, and 186Re;

(v) radionuclides that decay by electron capture or internal electron conversion, e.g., 77Ge, 103Pd, 109Sb, 131Cs, 193Pt, and 197Hg.

The characteristics of the radionuclide must ensure, on the one hand, selective and irreversible damage to the target tissue, and on the other hand, minimal damage to the excretory organs and healthy tissue adjacent to the tumor.

Thus, the choice of a particular type of radiation is influenced, among other things, by parameters such as tumor size and heterogeneity, tissue types, and exposure to radiation risk. The particle range must be optimized to irradiate the entire target volume while minimizing the irradiation of healthy tissue. This ideal range varies widely because target characteristics vary widely between diseases, individual patients, and individual targets.

2.2 Targeting Mechanisms

The success and safety of the use of radiopharmaceuticals in clinical oncology largely depend on the possibility of ensuring the most accurate delivery of a source of ionizing radiation to an oncological tumor with minimal impact on healthy tissue. That is why labeled molecules for targeted delivery must have high specificity (selectivity of interaction with a certain type of receptor), binding affinity and selectivity (selectivity of action on certain organs and tissues).

If specificity is responsible for the degree of affinity of the ligand for the receptor and depends on correct identification of the target carrier molecule, then selectivity is primarily associated with the degree of presence of the target (target, receptor) in the cancer tumor compared to normal tissue.

Therefore, an important task when creating RPs is correct choice of the target. The ideal target is a receptor that is overexpressed in the malignancy but is not expressed or has very low expression in physiological tissues. In addition, the receptor must be readily accessible and therefore preferentially expressed on the cell membrane or have an extracellular portion.

Such targets include cell surface glycoproteins and transmembrane glycoproteins (such as antigens of a cluster of differentiation (CD), folate receptors), glycolipids or phospholipids (e.g., disialoganglioside GD2, phosphatidylserine), carbohydrates (e.g., lectins), cell surface receptors (e.g., G protein-coupled receptors), integrins (e.g., αvβ3), growth-factor receptors (e.g., EGFR and VEGFR), transporters (e.g., LAT1, norepinephrine transporter), or enzymes (e.g., matrix metalloproteinase).

In recent decades, it has become obvious that the microenvironment (accounting for ~90% of all tumor tissue) of solid formations plays an important role in the emergence, development, metastasis of cancer tumors, as well as their resistance to drug exposure. Thus, stromal cells and other components of the extracellular matrix (tumor-associated fibroblasts, macrophages, adipocytes, tumor-infiltrating lymphocytes, endothelial cells of the new vasculature, etc.), as well as changes in metabolic processes (hypoxia, acidity, increased metabolism) may contain valuable prognostic and pathophysiological information and are considered as an informative marker and/or therapeutic target for targeted radionuclide diagnostics and therapy [16].

Depending on the targeting mechanism, the following areas of targeted nuclear medicine are distinguished (Fig. 2).

Fig. 2.
figure 2

Targeted nuclear medicine: advantages and main directions.

Full size image

1. Peptide-receptor radionuclide diagnostics and therapy, based on the lock-and-key fixation of target molecules on regulatory peptide receptors, most of which belong to the G-protein family: somatostatin receptors, gastrin-releasing peptide receptor, neurokinin receptors, cholecystokinin receptor, and a number of others.

Synthetic peptides with no more than 50 amino acids and possessing high stability and binding affinity are used as targeting ligands.

This method has been successfully used in the treatment of neuroendocrine tumors (NETs) in many nuclear-medicine centers [61]. For the visualization somatostatin receptors and assessment of the degree of expression, 111In-DTPA-Octreotide and 68Ga-DOTA-TOC are used, and for subsequent therapy, 90Y- and 177Lu-DOTA-TOC(DOTA-TATE) and their analogs are used [62].

2. Radioimmune diagnostics and therapy. The mechanism of the attachment of antibodies to antigens involves the production of specialized protective proteins (antibodies) by the immune system in response to a large number of antigens expressed on the surface of tumor cells and various components of the stroma [63]. Two radioimmunoconjugates capable of targeting the CD20 antigen have been approved by a number of national regulatory authorities: 131I-tositumomab and 90Y-ibritumomab tiuxetan. The clinical results of these drugs have shown significant efficacy and long-term effects in the treatment of non-Hodgkin lymphoma [64].

3. Radioligand diagnostics and small molecule-based therapies. Due to their relatively low molecular weight and increased lipophilicity, these ligands have higher pharmacokinetic parameters compared to antibodies or peptides. In addition, they are characterized by more cost-effective production and the ability to label with radioactive isotopes under more stringent conditions (higher temperature and wider pH range). The most well-known are low-molecular-weight PSMA inhibitor conjugates (PSMA-11, PSMA-617, PSMA I&T, etc.), which are used in the diagnosis and therapy of prostate cancer. As a rule, they are used as diagnostic radiolabels 68Ga and 64Cu, and for therapeutic purposes, 177Lu, 225Ac, and 213Bi [65–70].

3 VECTOR CARRIER MOLECULES

Targeting ligands such as antibodies, their fragments and derivatives, scaffold proteins, peptides and small molecules, are used as target carrier molecules. They can also be used for the targeted delivery of radiolabeled nanoparticles (a combination of active and passive targeting strategies).

3.1 Antibodies and Their Derivatives

The use of monoclonal antibodies (MAbs), both as independent units and as part of radiopharmaceuticals, is widely known in the treatment of various malignant neoplasms.

However, despite the undoubted clinical effectiveness, the use of these targeted PSs together with radionuclides has a number of limitations.

The main disadvantage of full-length antibodies is their large molecular weight (150 kDa). Due to the continuous process of radionuclide decay, including until the antibody binds to the corresponding antigen, healthy tissues are irradiated; therefore, it is extremely important to minimize the period of free circulation of radioconjugates. Due to their large size, MAbs are characterized by a long circulation time in the blood and reduced diffusion into tumor masses, which can lead to the accumulation of radionuclides in critical organs, especially the liver.

Thus, the main problem with the use of radiolabeled MAbs is their slow pharmacokinetics, leading to high doses of radiation to intact tissue, including radiosensitive bone marrow [64]. That is why, despite the active use of MAbs in the treatment of radiosensitive hematological tumors, the use of RPs for the treatment of solid tumors raises many questions.

Modern protein-engineering technologies make it possible to solve the above problem and accelerate clearance, extravasation, and penetration into tumor tissue by converting intact MAbs into smaller antibody derivatives. The latter consist of monovalent fragments, such as single-domain antibodies (sdAbs), diabodies, minibodies, protein scaffolds, and more complex bispecific antibodies (bsAbs), which allows them to be significantly reduced in size (15–110 kDa).

Small antibody fragments (less than ~70 kDa) can dramatically accelerate clearance and provide high-contrast images within 4–8 h, but at the same time lead to negative consequences associated with changes in the excretion pathways of RPs. Bypassing the hepatobiliary system, such antibody fragments are reabsorbed in the kidneys, which are less radioresistant compared to the liver.

A promising approach that can reduce the negative effects of radiation on the kidneys and reduce hematotoxicity due to the long-term presence of target molecules in the blood is pretargeted radioimmunotherapy [71]. At the beginning of the procedure, the patient is injected with a targeting vector designed to bind to the target antigen. After its accumulation in the tumor and the excess has been largely eliminated from the blood, an additional agent is introduced, which is a small molecule or short peptide labeled with a radionuclide. Upon collision with the targeting vector, ligation occurs between the two molecules, leading to the formation of a radioimmunoconjugate in vivo. Due to the small size of the secondary agent, it has favorable pharmacokinetic properties, hence the remaining radiolabeled small molecules are rapidly cleared from the body. Sometimes an additional “cleaning” step is introduced to remove the unbound targeting vector from the bloodstream before injecting the radiolabeled small molecule. After injection of the cleaning agent, the molecule quickly binds the target vector, and the newly formed complex is excreted through the liver.

3.2 Antibody Analogues

Antibody analogues based on alternative scaffold proteins (ASPs) are becoming increasingly widespread in biomedicine and biotechnology, due to the ease of their design and the possibility of obtaining various modifications by chemical synthesis or low-cost bacterial production [72]. They are modified natural polypeptides or fragments thereof, devoid of certain disadvantages such as large size, the need for closure of disulfide bonds, a tendency to form aggregates, reduced affinity, etc. In addition, ASPs are able to restore their properties and high affinity for molecular targets after thermal or chemical denaturation. This makes it possible to obtain labeled compounds at high temperatures (up to 95°C) from acidic or alkaline solutions (in the pH range 3.6–11.0). These include chemically synthesized aptamers (~5–30 kDa) and knottins (~4 kDa), as well as DARPins (~14–18 kDa), affimers (~12–14 kDa), and avimers obtained by phage display and bacterial expression (~4 kDa), monobodies (~10 kDa), anticalins (~20 kDa), finomeres (~7 kDa), and affibodies (~6.5 kDa). The small size of proteins (2–20 kDa) allows for easier penetration into tissues for binding, but this also results in rapid renal filtration and in some cases additional protein-engineering techniques may be required to increase the plasma half-life.

The first scaffold protein intended for targeted radionuclide therapy was the domain scaffold of protein A from the bacteria Staphylococcus aureus, i.e., affibody [73]. Its small size (58 amino acids) ensures good penetration of the molecule into a tumor and rapid removal of the unbound drug from the blood.

Preclinical studies have demonstrated successful targeting of the affibody to tumors, as well as a several-fold higher tumor-to-blood label distribution ratio than MAbs at 72 h post-administration [74, 75]. Today, a number of radiopharmaceuticals based on HER2-binding affibody molecules (for example, ABY-002 and ABY-025) labeled 68Ga and 111In, are undergoing phase two and three clinical trials, demonstrating good biodistribution and high contrast resolution within 2 to 24 h after injection [76].

Another promising type of ASP is a scaffold protein consisting of four or five repeats of 33 amino acids, a β sheet and two parallel α-helices, i.e., DARPin [77]. This protein, consisting of 130–160 amino acids, has a high affinity for the epidermal growth factor receptors EGFR and HER2 [78], as well as the membrane intercellular adhesion protein EpCAM [79] and is of undoubted interest for the molecular imaging of various neoplasms, primarily ovarian cancer [80] and triple-negative breast cancer [81].

3.3 Peptides

Targeted peptides are considered as an alternative means for the delivery of diagnostic agents and/or drugs. The peptides are small in size (from 1 to 15 kDa), due to which they diffuse well into tissues, are not immunogenic, are quickly cleared from the blood, and are extremely well tolerated by patients.

Another advantage of using peptides is the ease and cost-effectiveness of synthesis, as well as the possibility of versatile conjugation with imaging agents and drugs, and nanoparticles for targeted delivery. Peptides provide additional promising treatment options for modern personalized medicine.

Many human tumors overexpress regulatory peptide receptors, most of which belong to the G protein-coupled receptor family: somatostatin receptors, gastrin-releasing peptide receptor, neurokinin receptors, cholecystokinin receptor, and several others.

Somatostatin receptors (SR, SSTR). For the diagnosis and treatment of NETs, RPs to somatostatin receptors based on the ligand agonists DOTA-TATE, DOTA-TOC, and DOTA-NOС, labeled with the radioisotopes 68Ga, 18F (for diagnostics) and 177Lu (for therapy), have proven themselves positive. A cohort study of patients with metastatic well-differentiated NETs showed that therapy with these drugs can increase overall survival (more than 40 months from the first procedure) and time to disease progression (median 23.9 months).

In 2018, the RP Lutathera (177Lu-DOTA-TATE) was approved by the Food and Drug Administration (FDA) for the treatment of NETs.

Despite the achievable internalization of the radionuclide in tumor cells when using SR agonist ligands, modern research is directed towards the search for promising antagonists capable of interacting with a large number of binding sites. Publications report the high potential of the OPS201 ligand (DOTA-JR11), which demonstrates in cell lines and preclinical studies a higher tumor uptake, longer tumor residence time, and an improved tumor/kidney dose ratio than 177Lu-DOTA-TATE. It was also shown that 111In-OPS201 is a good candidate for breast-cancer diagnosis.

Bombesin receptor (gastrin releasing peptide) (BR, GRPR). BR overexpression is found in many different tumors, including small-cell lung cancer, breast, pancreatic, or prostate cancer. Targeting BR is a good imaging and treatment strategy for patients with ER-positive breast cancer. New radiopharmaceuticals are being developed based on peptide antagonists of BR, for example Neo-BOMB1, labeled with 68Ga, 111In, and 177Lu. PET/CT scans of two patients with prostate cancer showed that the use of 68Ga-NeoBOMB1 provides the high-contrast visualization of pathological lesions, and research is currently ongoing into the prospects for using this RP for theranostic purposes.

Neurokinin (or tachykinin) receptors (NR, NK). The overexpression of NR is observed in primary malignant gliomas. Substance P (neuropeptide of 11 amino acids of the tachykinin family, SP) is an endogenous ligand for PH receptor type 1 and appears to be an interesting agent for targeting brain tumors. Initial results from some pilot studies with 90Y-SP and 177Lu-SP demonstrated the high potential of this drug for imaging and therapy.

Given the growing interest in α-emitting radionuclides, a series of experiments was conducted to evaluate the effectiveness and safety of SP labeled with 213Bi, 225Ac, and 211At. The use of from 1.4 to 9.7 GBq [213Bi]Bi-DOTA-SP demonstrated median progression-free survival of 5.8 and 16.4 months overall. Additional [82, 83] preclinical and clinical studies (phase 1) of [225Ac]Ac-DOTA-SP and [211At]At-DOTA-SP are currently underway.

Chemokine receptor 4 (CXCR4). Together with the chemokine CXCL12, which is produced mainly in bone marrow, the lymph nodes, lungs, heart, thymus, and liver, CXCR4 forms the CXCL12-CXCR4 signaling axis, which is involved in the homeostasis of the adult hematopoietic system and adequate response of the immune system. Currently, two radiolabeled peptide imaging molecules that are ligands for CXCR4 have been tested in patients: 68Ga-Pentixafor and 68Ga-NOTA-NFB. 68Ga-Pentixafor showed a very good pharmacokinetic profile and rapid clearance. CXCR4 is also a good therapeutic and theranostic target. The therapeutic peptide agent for CXCR4 is Pentixather. Pentixather, labeled with 90Y and 177Lu, has confirmed its effectiveness in the treatment of patients with multiple myeloma.

Integrins αvβ3 and αvβ5 are used as indicators in breast cancer because they signal cell growth, including malignancy, metastasis, and cancer-induced angiogenesis. Radioactively labeled structures with a core of arginylglycylaspartic acid (Arg-Gly-Asp, RGD), which has high affinity and selectivity for αvβ3 and αvβ5 integrins, are used as targeting ligands. Examples include the following promising RPs: 99mTc-EDDA/HYNIC-E-[c(RGDfK)]2, 68Ga-BBN-RGD, 68Ga-TRAP-(RGD)3, etc.

3.4 Microspheres and Nanoparticles

Recent advances in the field of nanotechnology can significantly expand the capabilities of nuclear-physics methods used in medicine. Nanoparticles that vary in size from 1 to several hundred nanometers, are used as radiosensitizers in radiotherapy (X-ray and hadron-beam therapy), as part of drugs for neutron-capture therapy, as well as for the targeted delivery of radioisotopes during radionuclide diagnostics and therapy [84]. In the latter case, radionuclides can either be encapsulated in nanoparticles or attached to their surface using chelators. Both inorganic (nanomaterials based on gold, silver, silicon, etc.) and organic nanoparticles (carbon nanotubes and polymers, liposomes, etc.) are used as carriers [85]. The review [86] describes examples of the use of radiopharmaceuticals based on nanoparticles labeled with 99mTс, 64Cu, 177Lu, and 223Ra, in preclinical and clinical studies.

The obvious advantages of using nanotechnology are the ability to absorb per unit of mass a larger amount of adsorbed substances (heavy-metal ions, pesticides, radionuclides) compared to peptides and antibodies), the possibility of combining active and passive targeting mechanisms, the ability to simultaneously include several toxic agents for the purpose of combination therapy (radiotherapy, immunotherapy, chemotherapy) [87], as well as the applicability of a number of inorganic nanoparticles (due to additional optical, magnetic, and other properties) for multimodal theranostics.

One of the most notable limitations is the rapid recognition of nanoparticles by the mononuclear phagocyte system and the complement system, which leads to the accelerated elimination of nanoparticles from the bloodstream, as well as high uptake by the liver and spleen. Another problem preventing active implementation in clinical practice is the limitation of the penetration of nanoparticles into tumors due to their size.

To solve the first problem in [88] a new approach has been proposed, based on the introduction into the body of antibodies against its own red blood cells. As a result, the immune system concentrates on removing spent red blood cells for several hours, not paying attention to nanoparticles, which makes it possible to extend the circulation time of the latter in the bloodstream up to 32 times. Another option for blocking the mononuclear phagocytic system is the preliminary administration of nonfunctional nanoagents [89–91], which are attacked by the immune system without causing a decrease in the hematocrit. This approach makes it possible to extend the half-life of functional particles by up to 18 times.

The second task, associated with increasing the penetrating ability of radiopharmaceuticals based on nanoparticles, can be solved through the joint introduction of a special recombinant protein that binds desmoglein 2, which is responsible for the structural adhesion of neighboring epithelial cells. Such a solution allows for the temporary and specific opening of tight junctions and promotes the effective accumulation of nanoparticles around and inside the malignant neoplasm [92, 93].

Modular nanotransporters are worthy of special attention, which allow, based on natural cellular transport processes, to provide the “smart” paced delivery of the radionuclide to the nucleus of cancer cells [94]. This delivery vehicle is most attractive for radioisotopes with Auger-electron emission, capable of causing significant damage in the immediate vicinity of the site of decay upon targeted delivery to the compartments of the nucleus (Table 1), without having a detrimental effect on neighboring normal cells.

CONCLUSIONS

One of the breakthroughs in medicine in general and in oncology, first of all, was the use of nuclear-physics methods, including those based on the introduction of various radioactive substances to the patient: radionuclide diagnostics and therapy.

Modern nuclear medicine plays a significant role in the implementation of the principles of personalized (or precision) medicine through the use of targeted carrier molecules that provide the selective delivery, accumulation, and retention of the associated diagnostic or therapeutic radionuclide directly into the cells of malignant tumors, which provides their effective diagnostic imaging or local radiotherapy without significant radiation damage to both nearby and distant organs and tissues.

Therefore, the correct choice of the target and the creation of a scalable technology for the synthesis of a carrier molecule with high binding affinity to it have become extremely important tasks in the development of modern radiopharmaceuticals.

Understanding the molecular genetic and metabolic mechanisms of the life of tumor cells, and the molecular processes of their interaction with stromal cells opens up great opportunities for the creation of effective RPs based not only on peptides and MAbs, but also on targeted scaffold proteins and nanoparticles through the smart paced delivery of a radionuclide to a given target, as well as combination therapy.