Molecular Therapy of Leukemia

Michael Andreeff, M.D., Ph.D.

Although some leukemias, in particular those in children, can be cured by chemotherapy, the majority of patients still succumb to their disease. Patients with Hodgkin’s disease and certain non-Hodgkin’s lymphomas can also be cured, but again a large number of patients, after having an initial response to chemotherapy, become resistant and die as a direct consequence of their malignancy. Many new drugs have been developed over the last decades but very few have shown effectiveness in patients who are refractory to other drugs. This phenomenon has been called "multidrug resistance" and a pump, localized in the cell membrane of many tumor cells, has been identified as eliminating some cytotoxic agents from leukemia cells. However, blocking this pump has not resulted in a major increase in the efficacy of chemotherapy in patients indicating the existence of other drug resistance mechanisms.

We have, therefore, extended our previous studies on multidrug resistance to find molecules that are universally effecting drug resistance. These studies have uncovered molecules that are directly involved in cell death, which is now recognized as an active process, not just a passive disintegration of the effected cells. In some diseases, these molecules have not only been implicated in drug sensitivity, but also in the disease process itself by keeping malignant cells alive that would otherwise die. The particular attractiveness and potential clinical advantage lies in the fact, that these molecules are expressed in the majority, if not in all tumors, and that their manipulation would render leukemia and lymphoma cells more sensitive to a large variety of drugs. Of particular interest is the group termed "Bcl-2 family" (B-cell leukemia and lymphoma-2). This family consists of at least 10 proteins, which either prevent or enhance cell death in response to chemotherapy. Bcl-2 itself strongly protects cells from being killed by radiation and chemotherapy. Another family member, Bax, sensitizes leukemic cells to a variety of drugs: we found that lymphoid leukemic cells that overexpress Bax by gene transfer become 2.8-fold to 7.7-fold more sensitive to chemotherapeutic agents. By comparison, "high-dose chemotherapy", which has major side effects for the patient, usually allows only a 2-fold increase in the amount of drug that can be given to a patient.

We, then, developed a genetic strategy to overcome Bcl-2-mediated drug resistance. We constructed molecules that selectively and specifically block the production of the Bcl-2 protein. These molecules are called "Bcl-2-antisense-oligonucleotides." In the past, these molecules were easily destroyed by the cells and therefore ineffective. In collaboration with Dr. Lopez-Berenstein from The University of Texas M. D. Anderson Cancer Center, we have succeeded in packaging these molecules into tiny droplets of lipids (fats), which greatly facilitates the uptake of Bcl-2-antisense into the target cells and then protect them from destruction by the cells’ enzymes.

In extensive experiments, we have demonstrated that this Bcl-2-antisense molecule kills up to 80% of leukemia cells by itself. This was also seen in cells expressing high levels of the above-mentioned multidrug resistance protein, demonstrating the superiority of this approach in overcoming multidrug resistance. When we combined antisense molecules with chemotherapy, cells that were resistant to chemotherapy could be completely eliminated.

Of great importance is our finding that the earliest human bone marrow stem cells do not express Bcl-2 and are therefore not targets for this treatment approach. This should result in sparing normal blood and bone marrow cells and exerting toxicity only against leukemia and lymphoma cells.

An early clinical trial has already been conducted in London using this approach, although not with the optimized system that we have developed. Lymphoma patients who were completely refractory to chemotherapy responded and patients who did not respond subsequently had responses to the combination of chemotherapy and antisense oligos. Importantly, no toxicity and no side effects were observed except for some irritation of the skin, where the molecules were infused. We have seen responses in lymphoma models and in prostate cancer models, underlining the potential universal utility of this novel strategy to treat tumors by genetic alteration of resistance mechanisms.

Based on our laboratory results and on the initial trial conducted in London, we are now proposing to bring this approach to the bedside at The University of Texas M. D. Anderson Cancer Center. We are proposing to treat patients with leukemias and lymphomas with Bcl-2-antisense oligos in an initial study to evaluate toxicity and effectiveness of this approach. Later studies will then combine this molecule with chemotherapy: we expect that significantly lower doses of chemotherapy will yield better responses than high-dose chemotherapy when combined with Bcl-2-antisense oligos.

While preparing for these clinical trials, we are testing related molecules to make tumor cells more sensitive, building on our own and other groups’ work. Recently, an antisense molecule to Bcl-2 was published that was shown to be 10 times more effective than the molecules used by us so far. We have synthesized this new Bcl-2-antisense molecule and are presently testing it in leukemias and lymphomas.

These developments require top priority and start-up funding, until sufficient clinical data has been produced to engage pharmaceutical companies.