A New Use for One of the Oldest “New” Drugs

With the profusion of new targeted agents entering the clinical arena, a report from the American Society of Hematology bears consideration.

The trial known as the SORAML trial enrolled 276 patients with newly diagnosed acute myelogenous leukemia. The patients were between the ages of 18 and 60. All patients received a standard chemotherapy regimen. The patients were then randomized to receive Sorafenib or placebo. Patients on the Sorafenib arm then remained on a maintenance therapy for twelve months.

While the achievement of complete remission was almost identical between the two arms at 59% and 60%, the event free survival demonstrably favored the Sorafenib group at 20.5 months versus 9.2 months. At three years of follow-up 40% of the Sorafenib group were well with only 22% of the placebo group still in remission. This corresponds to a three-year relapse free survival of 38% for placebo and 56% for Sorafenib (P=0.017).

The results are of interest on several levels.
1.    Sorafenib a multitargeted tyrosine kinase inhibitor was approved in December 2005 for the treatment of renal cell carcinoma. This makes Sorafenib one of the first targeted agents to achieve FDA approval.

2.     Sorafenib has many modes of action and it is not entirely clear which of its functions were responsible for the superior survival in this AML study.

3.    Sorafenib’s approval reflects a rather convoluted and interesting history. When first developed the drug was designed to target the oncogene B-Raf. As a result the drug was introduced into early clinical trials for the treatment of advanced melanoma, a disease known to be associated with B-Raf mutation. As the drug proved ineffective, it appeared unlikely to gain FDA approval. That is, until it showed cross reactivity with VEGF pathway associated with tumor cell vascularity. A successful trial published in the New England Journal of Medicine then led to the approval.

Now, nine years later this old new drug has gained new life. This time in acute myelogenous leukemia.

The term “dirty drug” refers to agents that target many kinases at the same time. Sorafenib is an example of a “dirty drug.” However it is Sorafenib’s “dirty drug” quality that led first to its approval and most likely now leads to its application in AML. This reflects the fact that Sorafenib may be inhibiting B-Raf signaling associated with the common mutation in Ras upstream of B-Raf or it may reflect Flt3 a secondary activity associated with Sorafenib.

Indeed B-Raf and Flt3 may not be upregulated in every patient, but could serve a function of permissive activity granting an additional survival signal to the AML cells as they go through induction therapy. These subtleties of drug effect may escape genomic analysis as the true “target” may not be mutated, upregulated or amplified. No doubt the investigators in this study will conduct gene sequencing to determine whether there is a driver mutation associated with the advantage reported in this clinical study. What will be intriguing is to determine whether that advantage is an abnormal gene functioning within these cancerous cells or possibly a normal gene functioning abnormally in these cancer cells. More to come.

The Case for the Metabolic Basis of Cancer Gains Traction

Researchers from the Huntsman Cancer Institute at the University of Utah reported an interesting finding with far-reaching implications.

In their study of the rare tumor known as alveolar soft part sarcoma (ASPS), they examined the well-established chromosomal translocation that occurs between chromosomes 17 and X. This results in the production of a fusion protein dubbed ASPSCR1-TFE3. Like other fusion proteins described in malignancies such as lymphoma, acute pro-myelocytic leukemia and chronic myelogenous leukemia, a novel function occurs when two disparate genomic elements are spliced together.

In this instance, the ASPSCR1-TFE3 gene product functions as a lactate transporter. Strikingly, every mouse in which the gene was up regulated developed a tumor. The locations of tumor, in the skull and near the eye, both represented areas of high lactate concentration. In humans, this tumor occurs in skeletal muscle, also associated with high lactate production.

Since 1930, when Otto Warburg first described increased glycolysis (preferential use of sugars) in tumor cells, investigators have pondered the implication of inefficient glucose metabolism in the face of adequate oxygenation.

Human metabolism relies upon mitochondrial function to efficiently liberate the maximum amount of energy in the form of ATP from each glucose molecule. Glycolysis occurring in the cell cytoplasm is highly inefficient and produces only 1/18 of the amount of ATP that a full molecule of glucose can produce through mitochondrial oxidative phosphorylation. Recent molecular biological studies have established that the preferential use of glycolysis may represent the cells need to direct glucose away from energy production and toward the creation of essential structures like amino acids, lipids, and nucleic acids. With the rapid turnover of glucose, cells produce an overwhelming amount of lactate, which is then transported out of the cell. At least this has been the working hypothesis over many years.

More recently, investigators have begun to examine how lactate metabolism may represent the interplay between stromal fibroblast cells and tumor cells. Indeed, many tumor cells are now known to increase lactate uptake reflecting increased lactate production by fibroblasts that have been commandeered in the tumor microenvironment.

Lactate uptake is under the control of a family of transporters known as monocarboxylate transporters, of which nine have been described. These are expressed differently in various tissues, have different affinities for lactate and transport in one direction or another. These processes appear to be under the control of the major regulator of oxygen metabolism known as HIF-1 alpha. As cancer cells adapt to a high lactate environment, they can survive in low oxygen tension.

The preferential use of lactate as a source of energy is contrary to many dictates of current metabolic research that suggest that tumor cells preferentially use glucose and have limited capacity to utilize non-glucose energy sources like the ketone bodies acetoacetate and beta-hydroxybutyrate. Substantial literature on ketogenic diets suggests that these ketone bodies deprive cancer cells of needed nutrition and energy. The current discovery by the Utah investigators, as well as interesting work conducted by researchers in Italy on the prostate cancer, provide a new angle on some of these principles of cancer metabolism.

As the investigators from Utah note, the alveolar soft part sarcoma is a rare tumor, but the implications of these findings could be profound, as they force us to re-think tumorigenesis and the metabolic basis of cancer.