Radioimmunotherapy of human tumours

Abstract

The eradication of cancer remains a vexing problem despite recent advances in our understanding of the molecular basis of neoplasia. One therapeutic approach that has demonstrated potential involves the selective targeting of radionuclides to cancer-associated cell surface antigens using monoclonal antibodies. Such radioimmunotherapy (RIT) permits the delivery of a high dose of therapeutic radiation to cancer cells, while minimizing the exposure of normal cells. Although this approach has been investigated for several decades, the cumulative advances in cancer biology, antibody engineering and radiochemistry in the past decade have markedly enhanced the ability of RIT to produce durable remissions of multiple cancer types.

Figure 1. Target antigens for radioimmunotherapy

Target antigens for radioimmunotherapy (RIT) are most commonly accessible antigens on the cell surface of the neoplastic or stromal cell, and less commonly molecules that may be abundantly produced in the tumour environment. The simple case is targeting antigens on myeloid cells, and to a certain extent lymphomas, which are freely circulating in the blood and bone marrow. Radiolabeled antibody therapy of B-cell lymphoma include the receptor for B-cell activating factor (BAFF-R), class II histocompatibility antigens (HLA-DR), surface immunoglobulin (sIg), and cluster designation antigens CD19, CD20, CD22, CD37, and CD45. Target surface antigens for RIT of myeloid leukaemias include CD33 and CD45. For the solid tumours, there is also induction of stroma, vascular, and inflammatory cell components; RIT in principle can be applied to any one of these components (for example, to fibroblast activation protein-α (FAPα)), and commonly used cancer cell membrane targets include: carcinoembryonic antigen (CEA), prostate-specific membrane antigen (PSMA), GD2 ganglioside antigen (GD2) and carbonic anhydrase IX.

Figure 2. Intrathecal RIT imaged quantitatively with PET imaging, using ¹²⁴I-8H9 antibody

Images illustrate localization to leptomeningeal tumour of the radioactivity over the course of 72 hours. (All images shown are sagittal images through the intrathecal space). Immediately after intrathecal injection via an Ommaya Reservoir [G], a two-hour image shows complete filling of the intrathecal space, with distribution throughout the CSF and progressive clearance at 24 and 48 hours, except at the tumour site. At 48 hours, there is focal uptake at tumour sites evident in the thoracic and lumbar spine (arrows).

Figure 3. Results of selected trials of radioimmunotherapy as part of front-line therapy for follicular lymphoma

(A) Overall response rates, partial remission rates (purple), and complete remission rates (blue) are indicated for eight studies utilizing: ¹³¹I-tositumomab alone; cyclophosphamide, doxorubicin, vincristine, and prednisone followed by ¹³¹I-tositumomab (CHOP-B); fludarabine followed by ¹³¹I-tositumomab (Flud-B); cyclophosphamide, vincristine, and prednisone followed by ¹³¹I-tositumomab (CVP-B); rituximab plus CHOP followed by ⁹⁰Y-ibritumomab tiuxetan (R-CHOP-Z); fludarabine plus mitoxantrone followed by ⁹⁰Y-ibritumomab tiuxetan (FLUM IZ); or fludarabine, mitoxantrone, rituximab, and zevalin followed by ⁹⁰Y-ibritumomab tiuxetan (FMR-Z). Data are graphed from the published studies. (B) Progression-free and overall survival of 90 eligible patients with advanced Follicular Non-Hodgkin’s Lymphoma treated with six cycles of CHOP chemotherapy followed by tositumomab/¹³¹I-tositumomab on SWOG protocol S9911.

Figure 4. Schemas for conventional and pretargeted radioimmunotherapy

(A) Conventional single-step radioimmunotherapy (RIT), with monoclonal antibody conjugated directly to radionuclide. (B) Multi-step pretargeted RIT (PRIT) using antibody-streptavidin (Ab-SA) conjugates, followed by radiolabeled DOTA-biotin. The tetrameric streptavidin molecule can bind four radio-DOTA-biotin moieties, amplifying tumour-targeted radioactivity. The 16-merous, N-acetylgalactose-containing “clearing agent” removes excess Ab-SA conjugate from circulation via hepatic asialoglycoprotein receptors prior to systemic delivery of radio-DOTA-biotin, improving tumour-to-normal organ ratios of absorbed radioactivity. (C) Bispecific antibody pretargeting for RIT using radiolabeled bivalent haptens and “affinity enhancement system.” Another PRIT strategy employs bispecific antibodies recognizing both tumour-associated cell surface antigen and radiolabeled bivalent hapten (e.g., histamine-succinyl-glycine [HSG]), which facilitates cooperative binding by linking two bispecific Abs together on the cell surface. (D) Modular IgG-scFv bispecific PRIT. Disulfide-stabilized scFv with ultra-high affinity for radiometal DOTA fuses to C-terminus of an IgG light chain to create an IgG-scFv bifunctional antibody, targeting a cancer-associated antigen (e.g., CD20, CEA). ScFv (C825) is affinity-matured by directed evolution/yeast surface display to produce 1,000-fold improved affinity for biotinylated DOTA-yttrium compared to parent antibody (2D12.5). (E) Bispecific antibody pretargeting using “dock and lock” technology with self-assembling protein kinase-A domains^, . A clever modification of bispecific antibody targeting approach uses molecularly engineered “dimerization and docking domains” containing self-assembling protein kinase-A motifs with engineered cysteine residues. Upon mixing, two anti-tumour Fab fragments and one anti-hapten Fab fragment spontaneously associate, leading to “locking” of the fragments into a covalent trivalent complex.