Antisense Gene Therapy Trials Underway in Patients With CML

Responding to the need for more efficacious and less toxic treatmentsfor chronic myelogenous leukemia (CML), researchers at the Universityof Pennsylvania are exploring a novel form of gene therapy. Byinterfering with the transmission of a crucial message, they hopeto prevent malignant cell growth without affecting normal hematopoieticcells.

The strategy relies on blocking the "sense" sequenceof a particular gene's messenger RNA (mRNA), using a complementary"antisense" sequence to prevent translation and/or enhancedegradation, said Selina Luger, MD, assistant professor in theHematology-Oncology Division, University of Pennsylvania MedicalCenter, Philadelphia. In this way, she said, the malignant cellis deprived of the necessary encoded protein.

The strategy differs from conventional gene therapy because thegenetic constitution of a given cell is not altered, Dr. Lugersaid at the 13th annual symposium of the Chemotherapy Foundation.Furthermore, she emphasized, since it does not rely on retroviralvectors, a potential source of toxicity is eliminated.

Dr. Luger explained that since every gene has its own sense sequence,the antisense sequence must be custom- designed for a given gene.The c-myb proto-oncogene has been identified as preferentiallyexpressed in primitive hematopoietic tissues and leukemic tumorcell lines.

Purging Bone Marrow

Using LR3001, a specific antisense oligodeoxynucleotide, disruptsc-myb, Dr. Luger said. In vitro, it has been shown to decreaseproliferation of CML cell colonies at doses that still allow fornormal hematopoiesis.

Based on these preclinical findings, her group has begun a protocolof autologous bone marrow transplantation in patients with chronicor accelerated phase CML. In an attempt to eliminate the malignantclone from the autograft without systemic exposure, LR3001 isused ex vivo to purge the marrow.

The protocol is open to patients under age 65 with normal hepatic,renal, and cardiac function. To date, eight patients have beentreated, with encouraging results.

Infusional Antisense Therapy

Because unmodified oligonucleotide is susceptible to nucleaseattack, its use in vivo would not be possible, Dr. Luger said.LR3001 has been chemically modified to prevent nuclease digestionand therefore increase in vivo stability while maintaining therapeuticefficacy.

A phase I clinical trial has been initiated employing the modifiedform of LR3001 as an infusional agent. The trial seeks to determinethe maximally tolerated dose and dose-limiting toxicity of LR3001,as well as to assess pharmacokinetics and pharmacodynamics. Thetrial is open to patients with accelerated phase CML or blastcrisis, refractory or relapsed acute leukemia, and those withmyelodysplasia with excess blasts.

The modified LR3001 is being administered as a 24-hour continuousIV infusion over 7 days. Depending on response, patients may receiveadditional cycles every 28 days.

The trial is still in its earliest stages. Thus far, Dr. Lugerreported, no significant toxicities have developed at any of thefive dose levels. It is too soon, she said, to assess clinicalresponse.