Phenylbutyrate Fact Sheet

What is Phenylbutyrate?

Sodium Phenylbutyrate ("PB") is a small molecule drug previously approved for a rare genetic disorder, hyperuremia. It was subsequently found to have a potent anticancer effect, exhibiting cytostatic and differentiating activities. The initial work on the use of PB in cancer therapy was undertaken by the National Cancer Institute.

Cytostatic compounds reduce or stop the growth of tumors. Differentiation causes the tumor cells to change. Such changes may convert a malignant cell into more normal cells or cause the cell to mature, which reduces or eliminates its ability to divide, and slows tumor propagation.

Most drugs used in treating cancer are cytotoxic, i.e. they are designed to kill tumor cells, but while doing so, they can also kill or damage fast-dividing normal cells, giving rise to an array of adverse side-effects. By comparison, PB is well-tolerated and exhibits only minor side-effects. It is orally bioavailable, so it can be administered in tablet form.

There is a strong indication of PB efficacy in Phase 1 and Phase 2 trials, especially in gliomas (a form of brain cancer), as well as other CNS, blood (MDS, AML, APL) and colon cancers. The National Cancer Institute has sponsored, and continues to sponsor PB clinical trials. There are four ongoing and three planned trials. Seven clinical trials have already been completed. While it has shown efficacy as a monotherapy, PB will likely be administered in conjunction with radiation and chemotherapy.

Mode of Action of Phenylbutyrate

While PB operates on a number of different cellular pathways, it appears that one of the main processes involves the inhibition of the enzyme histone deacetylase (HDAC). Histone deacetylase inhibitors (HDACi) can have a number of anticancer effects including inhibition of cell proliferation, and cell cycle arrest potentially leading to apoptosis (cell death). While normal cells survive HDAC inhibition, HDACi have been shown to cause tumor growth inhibition in animal models. For example, PB has been shown to cause apoptosis and tumor remission in liver tumor xenografts.

There is currently significant interest and excitement regarding the potential of HDACi compounds for the treatment of cancer, and PB is one of the lead compounds in this area. PB’s other modes of action may include the inhibition of DNA methylation, activation of peroxisome proliferator-activated receptors, increased gap junction communication, and radiosensitization, all of which may assist in its antitumor activity when used alone or in combination with other agents.

Development Status

Access is developing BP in collaboration with its partner, Virium Pharmaceuticals, Inc. The next step in the development program for BP will be a Phase 2 clinical trial in patients with glioblastoma multiforme (GBM), an advanced and typically fatal form of brain cancer. The trial is presently planned to be undertaken by a group known as the New Approaches to Brain Tumor Therapy (NABTT) CNS Consortium (http://www.nabtt.org). The primary objective of NABTT is to improve the therapeutic outcome for adults with primary brain tumors. The NABTT consortium is one of two nationwide that are funded by the National Cancer Institute. Access and Virium anticipate the Phase 2 GBM trial will commence 4Q07.

Clinical Data for Phenylbutyrate

PB has been studied in a number of Phase 1 and Phase 2 clinical trials. Results on several of the most significant of these studies are summarized in this section:

• Oral PB was administered to 23 patients with recurrent malignant gliomas. Four dose levels (9, 18, 27, and 36 g/day) were studied. All PB doses were well tolerated. Fatigue and somnolence were the dose limiting toxicities, and the maximum toloerated dose (MTD) was determined as 27 g/day. One patient had a complete response of at least 5 years duration, and there were 5 patients exhibiting stable disease.
• Oral PB was given to 28 patients with a variety of solid tumors. MTD was 27g/day. PB was well tolerated. 7 patients had stable disease for more than 6 months.
• Phase 1/2 trial of fluorodeoxyuridine, dosed with 24-hour continuous intravenous infusion with dose escalation, in combination with PB (120 hour continuous intravenous infusion at a fixed dose 410 mg/kg/d x 5), repeated weekly. The combination was well tolerated. Three out of four patients who completed at least 8 weeks of treatment had stable disease.
• PB was administered by continuous infusion for 120hr primarily in hormone refractive prostate cancer patients. The drug was again well tolerated with some patients demonstrating stable disease.

There have been two additional reports in which patients treated with PB-treatment achieved a complete response:

• A GBM patient treated previously with radiation, Procarbazine–CCNU–Vincristine (PCV), then BCNU/Cisplatin was demonstrating disease progression prior to PB therapy. PB therapy resulted in a complete response, with the patient demonstrating a KPS functional score of 100% 4 years post PB therapy.
• An acute promyelocytic leukemiapatient that had proven clinically resistant to treatment with all-trans-retinoic acid alone achieved a complete response after PB was added to the treatment regiment. It was felt that PB restored sensitivity to the anti-leukemic effects of all-trans-retinoic acid.

The NCI lists over a dozen completed and ongoing trials involving PB:

• Phase 1/2 Study of Fluorouracil, Phenylbutyrate, Indomethacin, and Interferon Gamma in Patients With Advanced Colorectal Adenocarcinoma
• Phase 2 Pilot Study of Phenylbutyrate Plus Azacytidine in Patients With Acute Myeloid Leukemia, Myelodysplasia, Non-Hodgkin's Lymphoma, Multiple Myeloma, Non-Small Cell Lung Cancer, or Prostate Cancer
• Phase 2 Study of Phenylbutyrate in Pediatric Patients With Progressive or Recurrent CNS Malignancy
• Phase 1 Study of Twice-Daily Intravenous Phenylbutyrate for Advanced Cancer
• Phase 1 Study of Phenylbutyrate as a 120-Hour Infusion in Refractory Solid Tumors
• Phase 1 Study of Phenylbutyrate for Myelodysplastic Syndrome and Relapsed Acute Myeloid Leukemia
• Phase 1 Study of Phenylbutyrate in Children with Refractory Malignancies
• Phase 1 and Pharmacokinetic Study of Phenylbutyrate in Pediatric Patients With Refractory Cancer
• Phase 1 Study of Phenylbutyrate Plus Tretinoin in Patients With Acute Promyelocytic Leukemia, Neuroblastoma, Non-Small Cell Lung Cancer, or Prostate Cancer
• Phase 1 Study of Azacitidine in Combination With Phenylbutyrate in Patients With Recurrent, Refractory, or Untreated Acute Myeloid Leukemia or Myelodysplastic Syndrome
• Phase 1 Study of Azacitidine and Phenylbutyrate in Patients With Refractory Advanced Solid Tumors
• Phase 1 Study of Phenylbutyrate and Tretinoin in Patients With Myelodysplastic Syndromes, Chronic Myelomonocytic Leukemia, or Acute Myeloid Leukemia
• Phase 1 Study of Phenylbutyrate by Continuous Infusion in Patients with Cancer

Preclinical Efficacy Data for Phenylbutyrate

PB has engendered a great deal of research interest as an anticancer compound because it can be effective with little or no toxicity. There are numerous publications which report on preclinical studies investigating PB’s mechanisms of action, anticancer activity, and its ability to compliment and enhance the activity of other anticancer compounds. This section contains a brief summary of some of these studies.

Initial interest in PB was generated when in vitro studies showed that it has cytostatic activity in tumor cells, and it induced apoptosis in human prostate cancer cells. Additionally, PB was shown to potentiate the cytotoxic activity of doxorubicin in multi-drug resistant (MDR) tumor cells, indicating the potential for this drug to work with other anticancer drugs to overcome drug resistance.

PB was also combined with 13-cis retinoic acid (CRA; another differentiation-inducing agent) on the theoretical basis that their activities in tumor cells could be synergistic. In vitro, it was shown that the combination inhibited cell proliferation and increased apoptosis in an additive manner, while the combination demonstrated good tumor growth inhibition in an in vivo model employing the G-tumor cell line, as shown in the following figure:

Addition of a cytoxic compound, paclitaxel, to the PB and CRA combination in mice with HCT116 human colon tumors, provided superior results to paclitaxel alone, and >90% tumor growth inhibition was achieved, as shown below:

Induced cell differentiation by PB was demonstrated in human glioblastoma cell cultures. A series of studies has provided compelling evidence that PB enhances gap junction communication between tumor cells in culture, which enables PB to work synergistically with other anticancer compounds by enhancing the intercellular transport of low molecular-weight compounds. PB induces up to 70% apoptosis in several of the cell lines tested and it operated synergistically with agents such as gemcitabine and ganciclovir in tumor cells.

In models of non-small cell lung cancer (NSCLC), a combination of PB with gemcitabine was found to initiate a number of apoptosis-inducing events. Importantly, these events appeared to overcome therapeutic resistance in NSCLC tumor cells. In vivo, PB proved to be well tolerated in combination with gemcitabine, and the PB-mediated sensitizing effects combined well with gemcitabine to substantially reduced tumor cell proliferation.

In vitro studies to determine whether PB in combination with a variety of cytotoxic compounds were additive or synergistic in Malgnanat B cells revealed that PB was additive with doxorubicin, chlorambucil, melphalan, fludarabine, carboplatin, and cisplatin, and synergistic (i.e. the effect is greater than the sum of the two individually) with cytarabine, topotecan, etoposide.

PB inhibited proliferation and induced apoptosis in glioma cells. In a separate study, PB was shown to inhibit the migration and invasiveness of malignant glioma cells, indicating that phenylbutyrate is a promising candidate compound for treating patients with malignant glioma.

PB enhanced the inhibition of cell growth in fluorodeoxyuridine-treated human colon carcinoma cells, which prompted the evaluation of this combination in the clinic, as described in the previous section.

The highly promising preclinical data generated on PB provided ample support for the evaluation of PB in human subjects (as summarized in the preceding section). Meanwhile, further potential benefits of PB therapy are being discovered. As an example, it was recently reported in a mouse model, that PB protects the heart from doxorubicin-induced cardiac injury which is the most significant toxicity of this important anticancer compound.