Inclusion:

• Histologically or Cytologically proven SCLC, CD 56+ small cell carcinoma of unknown origin, or CD56+ non-pulmonary small cell carcinoma
• Relapsed disease; defined as patients with an initial response (partial or complete) to first-line therapy, then relapse more than 3 months after completion of last chemotherapy.
• Patients must have received no more than 3 prior chemotherapy regimen.
• Patients must have measurable disease defined as: Lesions that can be measured in at least one dimension according to RECIST
• Predicted survival of 3 months or more
• Zubrod performance status 0-2
• Patients must not have received chemotherapy or radiation therapy within 4 weeks of study entry, nor have planned surgery.
• Absolute neutrophils greater than or equal to 1.5 x 10^9/l, hemoglobin greater than or equal to 9g/dl and platelets greater than or equal to 100 x 10^9/l.
• Creatinine less than or equal to 1.5 times the upper limit of normal
• AST/ALT less than or equal to 3 times the upper limit of normal without liver metastases; less than or equal to 5 times the upper limit of normal with liver metastases and bilirubin less than or equal to 1.5 times the upper limit of normal.
• Patients must have normal thyroid function (patients receiving thyroxin replacement therapy who are biochemically euthyroid may be enrolled).
• Women of childbearing potential must provide a negative pregnancy test at screening and use adequate contraception in the opinion of the investigator, for the duration of study.
• Patients must be capable of understanding the nature of the trial and must give written witnessed informed consent prior to any screening procedure.

Exclusion:

• Significant residual neurological or cardiac toxicity (grade 3 or 4) following previous chemotherapy
• Patients who are concurrently receiving other anti-neoplastic treatment (chemotherapy, radiotherapy, or immunotherapy including steroid therapy).
• Myocardial infarction within 6 months of study entry, unstable angina pectoris, uncontrolled congestive heart failure, uncontrolled arrythmia, severe aortic stenosis, a history of multiple sclerosis, or other demyelinating disease, Eaton-Lambert Syndrome, history of hemorrhagic stroke, any CNS injury with residual neurologic deficit, ischaemic stroke within the last 6 months, current known herpes zoster (shingles), or cytomegalovirus infection, or a history of recurrent infections with these viruses, chronic alcoholism, serious concomitant infection, or any other concomitant illness considered significant enough to interfere with the study outcome.
• Other investigational agents must not be taken during the study or within 4 weeks of study entry.
• Previous monoclonal antibody therapy
• Patients must not have known central nervous system metastases
• Previous malignancy with < 5 year disease free interval from the last therapeutic intervention, except for adequately treated basal cell carcinoma of the skin or carcinoma in situ of the cervix.
• Patient unwilling or unable to tolerate and comply with the requirements of the study.
• Pregnant or lactating females.