Factors affecting chemistry of reduction-mediated 99mTc-labelling of monoclonal antibodies and polyclonal human immunoglobulins.

In developing a new method for preparing a radiopharmaceutical for clinical investigation, a thorough understanding of reaction stoichiometry is crucial in optimizing the labelling chemistry. Factors determining labelling efficiency of the 2-mercaptoethanol (2-ME)-mediated 99mTc-labelling of antibody molecules were elucidated using anti-tumor monoclonal antibodies of different IgG subclasses (i.e. IOR-CEA(IgG1), M170(IgG1), 3F8(IgG3) and EMD (IgG2a)) and polyclonal human immunoglobulins (Sandoglobulin). Antibodies which were sensitive to 2-ME reduction (i.e. required 500-1000 molar excess of 2-ME) could tag 99mTc with high efficiency since they possessed abundant reactive sites (i.e. sulfydryl groups) for 99mTc binding. Reduction sensitivity of antibodies was unlikely to be affected by IgG subclass and could be rated as follows: Sandoglobulin > IOR-CEA > 3F8 > M170 > EMD. Concentrations of the reduced antibodies for effective labelling appeared to be related to the reduction sensitivity, i.e. 0.2, 0.4 and 0.6 mg/ml were required for labelling of IOR-CEA, 3F8 and M170 respectively. In addition, susceptibility to 2-ME reduction seemed to reflect the rate of antibody labelling. For 2-ME resistant molecules, i.e. M170 and EMD, successful labelling could be achieved by using a slow 99mTc reducing agent such as SnCl2 instead of SnF2 which reacted more rapidly. Since 2-ME generates reactive sulfhydryl groups that are distal to antigen binding sites, the immunoreactivity of the modified antibody was not affected by the effect of reduction.

Photodynamic therapy in management of head and neck cancers and precancerous lesions.

Photodynamic therapy (PDT) is a new form of cancer treatment with low morbidity. In this study, PDT was evaluated for its effectiveness in management of recurrent or widespread precancerous lesions, primary cancers in inoperable sites, recurrent or residual cancers which were refractory to radiotherapy and chemotherapy, and advanced tumors in the head and neck. Fifty-one patients were treated over a period of 5 years. A 91.67 per cent complete response rate was observed for T1 tumors (primary and recurrence) with a recurrent rate of 27.27 per cent. Nasopharyngeal carcinoma was highly responsive to PDT since all T1 and T2 tumors responded completely. This was in contrast to cancers in the soft palate which failed in most cases possibly due to inadequate light dose distribution. PDT was remarkably effective in curing premalignant diseases (100% complete response rate). Postoperative PDT was equally effective in treating the microscopic residual malignancy. For advanced tumors, PDT in adjunct to conventional modalities could induce complete response in 5 out of the 10 patients and resolve symptoms in 4 cases. The mean follow-up time for this series was 28.3 months (range 3-66 months). In conclusion, PDT is a useful modality for the treatment of head and neck tumors and precancerous lesions presenting in forms or under conditions that posed considerable difficulties in management by conventional approaches.

Photodynamic therapy for residual or recurrent cancer of the nasopharynx.

Adequate or effective treatments are not always available for most recurrent or residual nasopharyngeal cancers (NPC). Photodynamic therapy (PDT) using hematoporphyrin derivative (HpD) was evaluated for its effectiveness in treating patients, who conventionally failed, with curative or palliative intent. Thirteen patients were treated from March 1994 to November 1998. PDT was given to eradicate tumor cells, debulk tumor mass for other treatment options, and to resolve obstruction. Long-term tumor control could be achieved in 6 patients with localized lesions at T1-T2 stages. The mean disease free survival time was 25.8 months (range 5-61 months). For tumors beyond T2 stage (7 cases), PDT in combination with chemotherapy, laser surgery or radiotherapy induced complete response in 1 out of 5 patients (survival time = 40 months) and partial response in the rest (survival time = 16-37 months). In two patients who refused or were in tolerable to further treatment, PDT yielded useful palliative results (i.e. resolve nasal obstruction and epistaxis). On an overall basis, the average survival time for these patients with relatively advanced diseases was 24.7 months (range 9-40 months). Our study demonstrated that HpD-PDT could effectively control locally recurrent or residual NPC at T1-T2 stages and offered good palliation for more advances. Combined PDT and chemotherapy seemed to prolong survival time for a period longer than 2 years in T3-T4 tumors.

Reduction-mediated 99mTc-labeling of antitumor monoclonal antibodies: effect of increasing specific activity on antibody binding kinetics.

Reduction-mediated 99mTc-labeling of antibodies has gained widespread acceptance in preparation of tumor imaging agents. Increased specific activity to enhance detection signals has raised the question of whether such an attempt would cause change in antibody binding kinetics. To answer this question, two antitumor monoclonal antibodies, i.e. IOR-CEA (IgG1) and EMD (IgG2a) were labeled with 99mTc to yield specific activities ranging from 549-4414 MBq/mg. Regression analysis of the binding data revealed that the binding kinetics of IOR-CEA were shifted from monovalent to bivalent binding upon increasing the specific activities. This phenomenon of affinity enhancement was confirmed by the dissociation study where we found soluble CEA had greater difficulty in extracting the cell-bound IOR-CEA labeled at higher specific activity. The bivalent bindings was further supported by the finding that IOR-CEA with higher specific activities delivered less than expected radioactivity to tumor targets despite their immunoreactivities being well preserved. For EMD, the kinetics seemed to be shifted from bivalent to monovalent interaction. At higher specific activities, adverse changes in immunoreactivity were recognized. Breakage of EMD into 99mTc-Fab fragments was likely to occur and was supported by the observation that EMD delivered more than expected radioactivities to target cells upon increasing specific activity. Precaution should be taken when one deals with high specific activity labeling since this might alter the antibody binding kinetics either favorably or unfavorably.

Photodynamic therapy in the treatment of head and neck cancers: a two-year experience.

This report describes the effectiveness of photodynamic therapy (PDT) in treating patients with widespread superficial premalignant and malignant lesions, primary and second primary, nonresponsive residual or recurrent cancers in head and neck. Twelve patients whose disease was at T1 stage responded completely. Severe dysplasia and field cancerous lesions involving a large area of oral mucosa (3 cases) also yielded excellent results. Average disease-free period was 13.6 months. The longest survival period was 2 years. All patients as mentioned are still alive without any relapse or recurrence. Combined PDT with other treatment was required to control T2-3 carcinomas. PDT adjunct, however, permitted the prescription of conventional treatment, i.e. radiotherapy or surgery, in a less morbid manner. No unacceptable treatment complications and skin photosensitivity were observed in this study. This indicated the potential role of PDT in the management of long-standing problems in head and neck cancers.

Differential radiosensitization of radioresistant human cancer cells by caffeine.

The effect of caffeine, the methylated xanthine, in sensitizing the lethal action of ionizing radiation in vitro was investigated in human cancer cells which were clinically known to be radioincurable. The tumor lines were hepatocellular carcinoma and colon adenocarcinoma. Plateau phase cultures, after absorbing doses of 2 Gy, survived at a rate of 56.30 per cent for colon cancer and at 66.05 per cent for liver cancer. Both lines were radiosensitized by caffeine but at different potencies. Noteworthily, hepatocellular carcinoma whilst less radiosensitive than colon adenocarcinoma was 4 times more susceptible to caffeine. The lowest effective caffeine concentration for liver cancer was 2 mM which slightly exceeded the anticipated lethal concentration in humans. Research on radiosensitizing effect of methylated xanthines on hepatoma system still remains intriguing. Future work should be pursued with the use of less toxic compounds, such as theobromine.