Halperin7e_CH29

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C H A P T E R 2 9  Radioimmunotherapy and Unsealed Radionuclide Therapy

diethylenetriamine penta- acetic acid [ 111 In-DTPA]) has been shown to be highly diagnostic for NETs. Affibody molecules 22 are classified as affinity ligands or scaffold proteins that are approxi- mately 7 to 9 kDa. These proteins are based on a 58 amino acid residue derived from staphylococcus protein A, which binds immunoglob- ulin. Various applications have been applied to affibody use, including radiolabeled targeting for therapy. Aptamers are single-stranded DND or RNA oligonucle- otides (8 to 12 kDa; 10 to 100 bases) that are selected in vitro from a random library by a process termed SELEX (systemic evolution of ligands by exponential enrichment). Aptamers are an attractive alternative to larger mAbs because they are chemically synthesized (do not require a biologic system such as mAbs), have a low cost of production, exhibit high affinities, have a small size, are rapidly cleared from the circulation, have an unlimited shelf life, exhibit rapid tissue penetration, and are nonimmunogenic. 23 Aptamers fold into unique sec- ondary and tertiary structures that not only exhibit high affin- ities for targets, but can also gain entry into target cells. The basic SELEX process consists of exposing a target to a random single-strand nucleic acid library, selecting candidates that bind to the target, repeating the process to further select can- didates with increasing affinity for the target, and sequencing the final candidates (Fig. 29.2). Variations of SELEX include selecting aptamers that bind cells, internalize into cells, or are delivered in vivo , respectively, termed cell-SELEX, cell inter- nalization SELEX, and in vivo SELEX. The basic configuration of an aptamer in a random library is a 5 ′ forward primer- binding site, a random region, and a 3 ′ reverse primer-bind- ing site. The selected candidates can therefore undergo PCR amplification. The typical complexity of random library is between 1 × 10 13–24 different random sequences. For example, if the random region consists of 30 possible nucleotides, then potentially 1 × 10 18 different aptamers will be available. In this particular example, 1 × 10 18 represents a different aptamer or “key” for every grain of sand in the entire world. This is an amazingly large and diverse screening tool. The major det- riment of using aptamers as targeting constructs for RIT is their short serum half-life (measured in minutes) secondary to nuclease degradation. Fortunately, research has shown that aptamers can be rendered nuclease resistant with the following modifications; 5 ′ -modified uracil, 4 ′ -thio, 2 ′ -fluoro, 5 ′ - α -P-borano, 2 ′ -amino, 2 ′ -deoxy-L-ribose, 5 ′ -phosphorothio, 2 ′ -methoxy, modification of bases. 29 Aptamers are amazingly versatile and can recognize nearly any type of target, from metal ions to whole cells and even entire organisms. Because of their chemical synthesis, aptamers seem to have a great chance of becoming a true tumor-specific and personalized delivery construct. Aptamers for cancer imaging or therapy are shown in Table 29.3. THE PHYSICS AND RADIOBIOLOGY OF RADIOIMMUNOTHERAPY RIT delivers radiation to the target tissue in a continuous, although declining, low–dose rate (LDR) fashion. Typical dose rates for RIT are in the range of 10 to 20 cGy per hour. The total dose delivered by RIT is low, in the range of 1,500 to 2,000 cGy, with an effective half-life of 24 to 72 hours. This can be compared to the high–dose rate (HDR) delivery of radiation by external beam radiation therapy (EBRT). EBRT typically will deliver radiation at a dose rate of 100 to 500

TABLE 29.2  TARGETING AND PHARMACOKINETICS OF INTACT IgG AND ANTIBODY FRAGMENTS IgG F(ab’)2 CH2 deletion Minibody Fab Diabody scFv MW 150 100 120 80 50 40–50 20–25 Serum half-life 2–3 d a 1 d Hours Hours Hours Hours 1 hr Metabolism Liver Liver Liver Liver Kidney Kidney Kidney Tumor uptake b ***** **** *** *** ** ** * Time to accretion Days 1 d Hours Hours Hours Hours 1 h

Section II

a Serum half-life for fully human IgG is approximately 23 days. b Tumor uptake values range from large (*****) to small (*). MW, molecular weight (kDa).

(b) the production of humanized or fully human antibod- ies (Fig. 29.1D). Chimeric antibodies retain murine V H and V L domains, whereas humanized antibodies retain murine CDRs. Fully human antibodies retain no murine components. Although the development of HAGA may not be important after a single dose of mAb in lymphoma patients, HAGA will have a greater detrimental impact for patients with solid tumors when treated with RIT. 10 It has been well proven that as the antibody changes from murine to humanized, the immunogenicity is lessened. This concept is important so that multiple doses or fractions of RIT can be delivered. Current technology allows for the production of fully human mAbs. The concept of adding a nuclear localizing signal to bring the mAb from the cell surface or cytoplasm into the cell nucleus is shown in Figure 29.1C. In this setting, Auger-emitting radio- nuclides will be effective. Another factor that is critical and influences antibody targeting and pharmacokinetics is antibody molecular size (Fig. 29.1B). As stated previously, RIT has been less success- ful for treating solid tumors than hematologic malignancies. This is largely because of the lack of radiosensitivity of epithe- lial tumors (compared to hematologic malignancies) and the poor penetration of mAbs into large tumors. The decreased penetration of 150-kDa antibodies into large tumors is a direct result of increased tumor interstitial pressure, an aber- rant tumor vasculature, and an abnormal tumor extracellu- lar matrix. 14–17 Additionally, 150-kDa antibodies need longer periods of time to accrete into tumors and have long serum half-lives. When radiolabeled, a long serum half-life of the targeting construct will increase exposure of the bone mar- row to radiation, which causes hematologic toxicity and limits the amount of antibody and radionuclide that can be given. To overcome some of these issues, methods have been used to generate antibody fragments of varying size and valency. These smaller fragments exhibit superior tumor penetration and clear more rapidly from the circulation. However, if clear- ance from the circulation is too rapid, this can further limit tumor penetration. Table 29.2 summarizes these general con- cepts for targeting constructs of various molecular weights. Although mAbs and their fragments represent the most commonly used targeting constructs for the delivery of a radionuclide to malignant tissue, other agents are either in use or are being investigated, consisting of peptides, 18–20 affi- body molecules, 21,22 and aptamers. 23,24 Nanostructures are also being investigated as carriers of radionuclides. 12,25,26 In their unmodified form, the targeting capabilities of nanostructures are rather nonspecific. 27 Peptides are amino acid sequences (typically 7 to 14 amino acids) that serve as opioids, hormones, sweeteners, protein substrate inhibitors, releasing factors, antibiotics, and cytoprotectors. 28 The overexpression of receptors that are specific for various peptides has led to the development of peptide-based radiopharmaceuticals. Somatostatin is one of the most common peptides and is overexpressed in a mul- titude of malignancies, including breast cancer, small cell lung cancer, medullary thyroid cancer, and neuroendocrine tumors (NETs). Somatostatin is rapidly degraded; however, its derivative, octreotide, is very stable. Octreoscan (indium-111