A New Generation of “Magic Bullets” for Molecular Targeting of Cancer

In this issue of Clinical Cancer Research, Santich and colleagues present a new class of radiopharmaceuticals, based on a self-assembling and disassembling (SADA) bispecific antibody (BsAb) platform (1). This unique approach utilizes tetramerization domain of the tumor suppressor protein, P53 (2). The tetramerization domain can be incorporated into other hybrid proteins and subsequently modified for affinity to obtain desirable (optimal) association and dissociation kinetics. In addition to the carefully engineered version of the P53 tetramerization domain–based SADA domains, the self-assembling proteins created by Santich and colleagues contain a humanized tandem single-chain variable fragment (scFv) BsAb with high affinity for ganglioside GD2 to aid tumor targeting (3), as well as DOTA for binding of chelated diagnostic or therapeutic radionuclides (4). Multivalency, high target avidity, and the physical size of tetramers (∼200 kDa) extend circulation and facilitate tumor binding as compared with anti-GD2 IgG-[L]-scFv–formatted BsAb. The approximately 50 kDa molecular weight of resulting P53-SADA-BsAbs secures fast, renal filtration–driven clearance after the unbound tetramers spontaneously disassemble. The fast clearance prevents the development of antidrug antibody (ADA) in immunocompetent tumor-free mice. Measurements of ADA titers in the plasma, following immunization and subsequent challenge with P53-SADA-BsAb or IgG-scFv-BsAb, showed significantly lower ADA titers in mice exposed to P53-SADA-BsAb. These results suggest that multiple doses of P53-SADA-BsAb might be given to an individual patient allowing repeated or fractionated treatment, if needed.

The success of radiopharmaceutical therapy (RPT) depends on tumor-specific delivery of therapeutic radionuclides (5). Positive outcomes of recent clinical trials led to FDA approval of new radiopharmaceuticals. However, the adverse effects, due to nonspecific binding and immunogenicity of radiopharmaceuticals, prolonged circulation of the radionuclides, and difficulties in assessment of radiation doses deposited in the tumor and normal tissues, limit its efficacy. Several approaches have been proposed to improve the pharmacokinetics of radiopharmaceuticals, including two- and three-step pretargeting. In a conventional two-step targeting, a nonradiolabeled, antitumor antibody with affinity to a radioactive payload, is administered and allowed to circulate throughout the body. After the time needed to bind to its target in the tumor and for clearance of unbound antibody, a radiolabeled small molecule is injected that has high affinity and specificity for the pretargeted IgG antibody. Radiolabeled small molecules have rapid in vivo pharmacokinetics and, if not bound to the tumor-localized IgG, are rapidly cleared from the body. However, due to relatively long clearance time of IgG and variable pharmacokinetics, it is difficult to assess the optimal time for injection of the radionuclides for individual patients. If the radioactivity is injected too soon, it binds to circulating antibodies resulting in prolonged exposure of normal tissues and undue adverse effects, such as myelotoxicity. In a three-step approach, IgG-scavenging agents are used to remove excess antibodies from circulation. A three-step approach adds other levels of complication. The scavenger-facilitated clearance of radioactivity might not be sufficient, leaving unbound IgG in the blood, or might interfere with tumor uptake if the amount of the injected scavenger is not optimized to the tumor burden and antigen density. Results of preclinical studies indicate that the new approach proposed by Santich and colleagues (Fig. 1), utilizing self-clearing molecules in the first step of targeting, might provide a promising alternative to these approaches.

The potential of P53-SADA-BsAbs platform in a two-step theranostic has been tested using ⁸⁶Y-DOTA as the diagnostic payload and ¹⁷⁷Lu- or ²²⁵Ac-proteus-DOTA (6) as the therapeutic payload. For PET/CT imaging of the tumor, ⁸⁶Y-DOTA was injected 48 hours following the administration of P53-SADA-BsAbs. The PET images obtained 18 hours later, showed superior tumor-specific accumulation of ⁸⁶Y as compared with those obtained with conventional two- and three-step targeting using IgG-27scFv-BsAb. These results were confirmed by quantitative ex vivo measurements of tissue uptake. The capacity of reliable noninvasive assessment of radioactivity delivered to tumor and to normal tissues is crucial for selection of the patients that might benefit from RPT and designing of an optimal treatment plan for individual patients, based on prediction of radiation dose delivered to the tumor and normal tissues by therapeutic radionuclides. Therapeutic potential of ganglioside GD2–specific P53-SADA-BsAbs combined with therapeutic radionuclide-loaded DOTA was assessed in neuroblastoma and small-cell lung cancer tumor–bearing athymic nude mice. Administration of increasing activity of ¹⁷⁷Lu-DOTA, following P53-SADA-BsAb, resulted in strong linear correlation between the injected dosage and tumor accumulation of radioactivity. Because the radioactivity in the blood and the kidneys remained low, the therapeutic ratio increased accordingly. Dose estimates indicated that P53-SADA-BsAb combined with a 15 MBq dose of ¹⁷⁷Lu-DOTA payload could deliver radiation dose of 50 Gy to the tumor, with less than 2 and 0.5 Gy delivered to the kidneys and blood, respectively. Because the increase of the injected radioactivity led to much higher increase of its tumor accumulation than in normal tissues, it was possible to safely deliver cytotoxic radiation doses to the tumor. Three weekly rounds of the two-step treatment (Fig. 1), using 55.5 MBq/dose of ¹⁷⁷Lu-DOTA as the therapeutic payload, resulted in complete response of neuroblastoma patient-derived xenografts (PDX) in BALB/c-Rag2^null IL2rγ^null mice. Because of higher relative biological effectiveness of alpha particles emitted by ²²⁵Ac, one round of the two-step treatment using 37 kBq of ²²⁵Ac-proteus-DOTA as the therapeutic payload was sufficient to control both neuroblastoma and small-cell lung carcinoma PDXs. In all cases no dose-limiting toxicities were observed, and no evidence of radiation damage was detected by histologic examinations of normal tissues.

It is noteworthy that the modular character of SADA makes it a highly flexible anticancer platform. Several kinds of human-derived oligomerization tags can be used and BsAbs can target different antigens in many human tumor types. In addition, SADA can be combined with a therapeutic load other than radioactivity in a two-step treatment or can even be directly linked to a cytotoxic agent for a one-step delivery.

The results of the preclinical tests reported by Santich and colleagues (1) indicate that the two-step RTP combining P53-SADA-BsAb with DOTA radioconjugates might provide an effective therapeutic approach for cancer treatment. Prospective trials of this agent are anticipated. This process will be facilitated by already secured patent protection and industrial sponsorship. Hopefully, the outcome of the upcoming first-in-human studies will keep and make stronger the promise of safe and effective PRT.

No disclosures were reported.