Weekly Administration of (bi-)Daily Oral Docetaxel in Combination With Ritonavir

Inclusion Criteria:

1. Histological or cytological proof of cancer
2. Patients for whom no standard therapy of proven benefit exist
3. Patients who might benefit from treatment with docetaxel, e.g. advanced breast, gastric, esophagus, bladder, ovarian cancer and non-small cell lung cancer, head and neck cancers, prostate cancer and carcinoma of unknown primary site.
4. Age \_ 18 years
5. Able and willing to give written informed consent
6. Able and willing to undergo blood sampling for pharmacokinetics
7. Life expectancy \_ 3 months allowing adequate follow up of toxicity evaluation and anti-tumor activity
8. Minimal acceptable safety laboratory values

   * ANC of \_ 1.5 x 109 /L
   * Platelet count of \_ 100 x 109 /L
   * Hepatic function as defined by serum bilirubin \_ 1.5 x ULN, ALAT and ASAT \_ 2.5 x ULN
   * Renal function as defined by serum creatinine \_ 1.5 x ULN or creatinine clearance \_ 50 ml/min (by Cockcroft-Gault formula).
9. WHO performance status of \_ 2
10. No radio- or chemotherapy within the last 4 weeks prior to study entry (palliative limited radiation for pain reduction is allowed)
11. Able and willing to swallow oral medication

Exclusion Criteria:

1. Patients with known alcoholism, drug addiction and/or psychotic disorders in the history that are not suitable for adequate follow up
2. Women who are pregnant or breast feeding.
3. Both men and women enrolled in this trial must agree to use a reliable contraceptive method throughout the study (adequate contraceptive methods are: condom, sterilization, other barrier contraceptive measures preferably in combination with condoms).
4. Concomitant use of MDR and CYP3A modulating drugs such as Ca+-entry blockers (verapamil, dihydropyridines), cyclosporine, quinidine, quinine, tamoxifen, megestrol and grapefruit juice, concomitant use of HIV medications; other protease inhibitors,(non) nucleoside analoga, St. Johns wort or macrolide antibiotics as erythromycin and clarithromycin.
5. Uncontrolled infectious disease or known HIV-1 or HIV-2 type patients
6. Unresolved (\>grade 1) toxicities of previous chemotherapy
7. Bowel obstructions or motility disorders that may influence the absorption of drugs
8. Chronic use of H2-receptor antagonists or proton pump inhibitors
9. Neurologic disease that may render a patient at increased risk for peripheral or central neurotoxicity
10. Pre-existing neuropathy greater than CTC grade 1
11. Symptomatic cerebral or leptomeningeal metastases
12. Evidence of any other disease, neurological or metabolic dysfunction, physical examination finding or laboratory finding giving reasonable suspicion of a disease or condition that contraindicates the use of an investigational drug or puts the patient at high risk for treatment-related complications.