Inclusion Criteria:

1. Age greater than 16 years old.
2. Signed informed consent.
3. During the phase I portion of the trial, any patient with a proven solid tumor for which no curative or standard treatment is available is eligible. However, for the phase II portion of the trial, patients are required to have medullary thyroid carcinoma that is unresectable or metastatic.
4. For the phase I portion of the protocol, there is no limit to the amount of prior therapy participants may have received. For the phase II portion of the trial, 0-1 prior regimens are allowed.
5. ECOG performance status must be 0-2.
6. Adequate hepatic, renal, and bone marrow function: Absolute neutrophil count greater than/equal to 1,500/uL; platelets greater than/equal to 100,000/uL; total bilirubin less than/equal to 1.5 X institution upper limits of normal (ULN); AST (SGOT)/ALT (SGPT) less than/equal to 2.5 X institutional ULN; Creatinine less than/equal to 1.5 mg/dL
7. Female patients of childbearing potential must have a negative pregnancy test within 7 days before initiation of study drug dosing. Postmenopausal women must be amenorrheic for at least 12 months to be considered of non-childbearing potential. Male and female patients of reproductive potential must agree to employ an effective barrier method of birth control throughout the study and for up to 3 months following discontinuation of study drug.
8. Patients may have received prior radiation treatment but the last fraction of radiation treatment must have been completed at least 4 weeks prior to entry on this study.
9. Patients may have been treated with surgery but the surgical intervention must have been done at least 3 weeks prior to entry on this study.
10. In the phase I part of the trial, measurable disease is not required. Radiographic and measurable evidence of disease is required for the phase II part of the trial. To be considered evaluable for complete or partial response, patients must have at least one measurable lesion as per the modified RECIST Criteria. If radiation was previously received, measurable disease must occur outside the previous radiation field, unless disease progression has been documented.
11. Both men and women and members of all ethnic groups are eligible for this trial.

Exclusion Criteria:

1. In previously treated patients, patients should not have received prior dacarbazine, imatinib mesylate, 5-fluorouracil, or capecitabine. This requirement does not apply to the phase I patients.
2. Uncontrolled intercurrent illness including, but not limited to, active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmias not well controlled with medication, myocardial infarction within the previous 6 months, or psychiatric illness/social situations that would limit compliance with study requirements.
3. Patients who have had chemotherapy within 3 weeks prior to entering the study or those who have not recovered from adverse events due to agents administered more than 28 days earlier.
4. Patients may not be receiving any other investigational agents, or have participated in any investigational drug study within 28 days preceding start of study treatment.
5. The teratogenic potential of this combination is currently unknown. Women who are pregnant or lactating are excluded.
6. History of any other malignancy in the last 5 years, except that patients with a prior history of in situ cancer or basal or squamous cell skin cancer are eligible. Patients with treated disease processes believed to be associated with MEN2, such as pheochromocytomas and primary hyperparathyroidism are allowed in the study.
7. Concomitant use of warfarin is not allowed. Low molecular weight and standard heparin use is allowed.