[177Lu]-Labeled [(R)-2-amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (CHX-A″-DTPA) conjugated monoclonal antibody L8A4 against epidermal growth factor receptor variant III (EGFRvIII)

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The biological characteristics, activating ligands, functions, and signal transduction pathways of the various transmembrane epidermal growth factor receptors (EGFRs) are described elsewhere (1-3). The EGFRs are known to regulate the growth, survival, differentiation, and migration of cells through the activation of an associated intracellular tyrosine kinase (TK) signaling pathway, and they are overexpressed in many malignant epithelial tumors (2, 3). Overexpression of the EGFR in tumors has been attributed to cellular amplification of the receptor gene, and this phenomenon may result in the production of a mutated receptor in the cell (2, 4). In addition, overexpression of the EGFR in tumors usually indicates a poor clinical prognosis for a cancer patient (4). The most common mutation observed in the receptor is the deletion of an extracellular domain segment of the EGFR, including the ligand-binding region, and this generates a variant known as EGFRvIII or de2-7 EGFR (2, 4). The generation, structure, functions, and role of EGFRvIII in tumor malignancy was reviewed by Gan et al. (5). Although EGFRvIII is nonresponsive to a ligand due to the absence of a ligand-binding site, it is constitutively active with a constantly operating downstream TK signal transduction pathway that appears to promote the development of a neoplastic phenotype, particularly for glioblastoma and to some extent for other cancers such as those of the prostate and the breast (2, 6).

Because the EGFR promotes and helps maintain transformed cells, several anti-EGFR antibodies that inhibit the activity of this receptor and small molecule drugs that block the downstream TK signaling pathway were developed and have been approved by the United States Food and Drug Administration for the treatment of certain cancers (2). The antibodies are designed to target the extracellular domain of the receptor, block ligand binding, and inhibit activation of the TK signal transduction pathway, which leads to downregulation of the EGFR on the cell surface. However, because EGFRvIII lacks the ligand-binding region on the extracellular domain, these antibodies cannot obstruct the constitutive mutant receptor activity (2). Hence, a monoclonal antibody (1), designated mAb806 and specifically targets the EGFRvIII, was generated and has been characterized in preclinical studies (7, 8). Subsequently, a chimeric form of the mAb (chAb), designated ch806, was developed and evaluated in a phase I clinical trial involving patients with cancerous tumors that overexpressed the EGFRvIII. Results from this trial indicated that ch806 can be a suitable biotherapeutic agent to treat cancers (4). Various studies performed with mAb806 or ch806 are described in separate chapters of MICAD.

Studies are also in progress with another anti-EGFRvIII mAb, L8A4, to develop a radioimmunotherapeutic (RIT) agent for the treatment of cancer (9, 10). In a recent study, the use of acyclic ([(R)-2-amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (CHX-A''-DTPA) and 2-(4-isothiocyanatobenzyl)-6-methyldiethylene-triaminepentaacetic acid (1B4M-DTPA) as well as the macrocyclic ligands (S)-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-tetraacetic acid (C-DOTA) and α-(5-isothiocyanato-2-methoxyphenyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (MeO-DOTA) was evaluated for the labeling of L8A4 with ¹⁷⁷Lu, which was considered suitable to generate an RIT agent against cancer (10). The characteristics of the various ¹⁷⁷Lu-labeled conjugates of L8A4 ([¹⁷⁷Lu]-CHX-A''-DTPA-L8A4, [¹⁷⁷Lu]-1B4M-DTPA-L8A4, [¹⁷⁷Lu]-C-DOTA-L8A4, and [¹⁷⁷Lu]-MeO-DOTA-L8A4) were compared to the characteristics of L8A4 labeled with N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB-L8A4) under in vitro (9) and in vivo (10) conditions.

This chapter describes results obtained from the in vitro and the biodistribution studies performed with [¹⁷⁷Lu]-CHX-A''-DTPA-L8A4 in athymic mice bearing subcutaneous U87MG.∆EGFR cell glioma xenograft tumors. Results obtained with [¹²⁵I]SGMIB-L8A4, [¹⁷⁷Lu]-1B4M-DTPA-L8A4, [¹⁷⁷Lu]-C-DOTA-L8A4, and [¹⁷⁷Lu]-MeO-DOTA-L8A4 are presented in separate chapters of MICAD (www.micad.nih.gov) (11-14).