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CAT 8015 – A Trojan Horse

Date: April 12 , 2007

by Chaya Venkat

Monoclonal Antibodies with Enhanced Firepower

Related Articles: Monoclonal Antibody Review

Trojan Horse

I have heard from a few members about the newly announced clinical trial using CAT-8015, an immunotoxin, to treat B-cell malignancies including CLL. You can read about the clinical trial itself by clicking on the link below. Here is a quick review of the drug and how it is thought to work: just in case you are facing therapy choices using this particular drug. This is often how we work at CLL Topics, addressing issues of interest to our members, sometimes triggered by feedback or questions we get from you.

Link: CAT-8015 Clinical Trial


A few years ago there was a great deal of buzz about the potential use of “Ontak” (the scientific name is denileukin diftitox) in CLL patients. Ontak was the first well-known immunotoxin drug. Now the NCI is looking at “CAT-8015” as the second in this class of drugs to treat CLL (and other B-cell cancers). While Ontak continues to be interesting in T-cell CLL (yes my friends, CLL comes in two flavors, the B-cell variety we are all so fond of, as well as a much rarer T-cell variety that we do not bother to discuss often on this website), Ontak has been a disappointment in treating B-CLL patients.


Br J Haematol. 2007 Feb;136(3):439-47

Phase II trial of denileukin diftitox (Ontak) for relapsed/refractory T-cell non-Hodgkin lymphoma

Dang NH, Pro B, Hagemeister FB, Samaniego F, Jones D, Samuels BI, Rodriguez MA, Goy A, Romaguera JE, McLaughlin P, Tong AT, Turturro F, Walker PL, Fayad L.

Department of Hematologic Malignancies, Nevada Cancer Institute, Las Vegas, NV 89135

This phase II study evaluated the safety and efficacy of denileukin diftitox, an interleukin-2-diphtheria toxin fusion protein, in relapsed/refractory T-cell non-Hodgkin lymphoma (T-NHL), excluding cutaneous T-cell lymphoma. Eligible patients received denileukin diftitox 18 microg/kg/d x 5 d every 3 weeks for up to eight cycles. Tumour staging was performed every two cycles and the primary endpoint was the objective response rate [complete response (CR) + partial response (PR)]. For 27 patients enrolled, median age: 55 years (range 26-80 years), 70.4% male, and mean prior therapies: 2.5 (range 1-6). Objective responses (six CRs, seven PRs) were achieved in 13 patients (48.1%), stable disease in eight (29.6%) and six (22.2%) had progressive disease. An objective response was achieved in eight of 13 patients (61.5%) with CD25(+) tumours (four CR/four PR) and five of 11 patients (45.5%) with CD25(-) tumours (two CR/three PR). Median progression-free survival was 6 months (range, 1-38+ months). Most adverse reactions were grade 1/2 and transient. No grade 4-5 toxicities were reported. Denileukin diftitox had significant activity and was well tolerated in relapsed/refractory T-NHL, with responses observed in both CD25(+) and CD25(-) tumours. Further studies of denileukin diftitox in combination with other agents are warranted in previously untreated and relapsed/refractory T-NHL.

PMID: 17233846

Cancer. 2006 May 15;106(10):2158-64

Phase II clinical studies of denileukin diftitox toxin fusion protein in patients with previously treated chronic lymphocytic leukemia

Frankel AE, Surendranathan A, Black JH, White A, Ganjoo K, Cripe LD.

Department of Medicine, Scott and White Hospital, Temple, Texas 76508

BACKGROUND: The safety and efficacy of the interleukin-2 diphtheria toxin fusion protein (DAB(389)IL2; denileukin diftitox) directed against the IL-2 receptor (IL-2R) was tested in patients with recurrent or refractory chronic lymphocytic leukemia (CLL).
METHODS: Denileukin diftitox was administered as 60-minute intravenous infusions for 5 days every 21 days at a dose of 18 mug/kg per day for up to 8 cycles. In total, 28 patients were treated in 2 multiinstitutional studies with similar eligibility criteria and treatment protocols. Twenty-two patients receive > or = 2 cycles of denileukin diftitox and were evaluable for response.
RESULTS: Twelve of 22 patients achieved reductions of peripheral CLL cells, with 5 of 12 patients achieving >80% reductions. Six of 22 patients achieved reductions in the size of lymph node on examination and computed tomography scans, and all 6 of those patients met the criteria for a partial or complete response that lasted > or = 2 months. Bone marrow biopsies before and after treatment confirmed a complete remission that lasted for 1 year in 1 patient. Overall, denileukin diftitox produced complete remission in 1 of 22 patients (4%) and partial remission in 5 of 22 patients (23%) for a total remission rate of 27%. Progression-free intervals in the responders were 2 months in 2 patients and 4 months, 6 months, 7 months, and 12 months in 1 patient each. Toxicities were moderate. No infections associated with immunosuppression were seen. There was no significant correlation of response or toxicities with the numbers of denileukin diftitox cycles received or with CD25 levels.
CONCLUSIONS: Follow-up studies will be required to identify predictors of response that may improve the response rate to denileukin diftitox in patients with CLL.

Copyright 2006 American Cancer Society

PMID: 16586495

What is CAT-8015?

Ontak and CAT-8015 are both based on monoclonal antibody technology. Monoclonal antibodies work by being able to target and bind to a specific marker on the cancer cells. By now you know that Rituxan and HuMax-CD20 target the CD20 marker present on all mature B-cells (and only mature B-cells), while Campath-1H targets the CD52 marker that is present on a number of immune system cells. Lumi (lumiliximab) targets CD23. All of these are examples of “naked” monoclonal antibodies, in that they do not carry any particular poison or radioactive material with them. All of these drugs attach themselves to their marker of choice on the surface of the target cell and try to stay stuck like glue. Cell-kill using naked monoclonals involves the body participating in the cell kill — by the cancer cell accepting suicide commands, by means of complement or other immune system cells lending a helping hand. This is perhaps the single biggest handicap of naked monoclonal antibodies — the fact that they need some help from the patient's own system to bring about the cell kill. For cancer patients who are not equipped with a full deck as far as their immune systems go, this may be a tall order. It might explain why single agent naked monoclonal antibodies rarely produce deep or lasting remissions in CLL. The remission rate is sharply lower in patients who have been through several rounds of prior chemo and their bodies are no longer up to the task of doing the heavy lifting in helping the monoclonals to kill the cells that have been targeted. Immunotoxins try to get around this hurdle by bringing their own fire-power to the party.

The Trojan Horse Approach

Basically, both Ontak and CAT-8015 are monoclonal antibodies (similar in that sense to Rituxan, Campath, HuMax-CD20, Lumi, etc.) but with the major difference that both Ontak and CAT-8015 carry their own destructive agent with them. In the case of Ontak the poison is a snippet of the diphtheria bacterium. In the case of CAT-8015, the poison of choice is pseudomonas bacterium. In both cases, the snippet it is a very carefully chosen bit of the full bacterium, not the whole enchilada. This makes sense: researchers are not trying to kill their patients by inflicting full fledged diphtheria or pseudomonas infections. Below is a table of recent immunotoxin drug candidates that have caused a stir in the research community.

Immunotoxins Under Investigation
Kreitman RJ. Immunotoxins for Targeted Cancer Therapy. AAPS Journal. 2006
Immunotoxin Agent Antigen Target Toxic Component Diseases
Ontak IL-2R Diphtheria


BL22 CD22 Pseudomonas

Hairy Cell Leukemia, B-CLL, NHL

LMB-2 CD25 Pseudomonas

NHL, leukemias

CAT-8015 CD22 Pseudomonas CLL, PLL, SLL

As you can see from the table above, Ontak targets the interleuken-2 receptor (IL-2R) that is present on T-cells and B-cells, much more so on T-cells than it is on B-cells. Perhaps that explains why Ontak was so much more effective in treating T-CLL than it was in treating our garden variety B-CLL.

CAT-8015 ( as well as its predecessor BL22) targets another marker, called CD22. Both IL-2R and CD22 markers share one feature that sets them apart from the CD20, CD52 and CD23 markers that we have discussed in the past, with reference to Rituxan, Campath and Lumi, respectively. When CD20, CD52 are tagged by their respective monoclonal antibody drugs, the markers stay attached to the surface of the cell, with the monoclonal dangling from them. But IL-2R and CD22 behave differently. When they are tagged by Ontak or CAT-8-15, the cell pulls in its horns, as it were: the marker and the monoclonal drug attached to it is pulled into the interior of the cell.

Now you may understand why I call this the Trojan Horse approach. The antibody component of the drugs such as Ontak and CAT-8015 are just a way for the drug to tag the cell, get entry into the cell itself, like the wooden horse the Greeks were thought to have used to get inside Troy. Once pulled inside, the cell is quickly killed by the poison that is attached to the monoclonal antibody. Here is an abstract that describes how BL22 (think of this as an earlier version of CAT-8015) worked in hairy cell leukemia, another member of the family of B-cell cancers.


Best Pract Res Clin Haematol. 2006;19(4):685-99

BL22 and lymphoid malignancies

Kreitman RJ, Pastan I.

Centers for Cancer Research, National Cancer Institute, National Institutes of Health, 9000 Rockville Pike, Building 37, Room 5124b, Bethesda, MD 20892-4255

BL22 is a recombinant immunotoxin containing a truncated form of the bacterial toxin Pseudomonas exotoxin A attached to an Fv fragment of an anti-CD22 monoclonal antibody. Its mechanism of action involves binding to CD22, being internalized into the target cell by endocytosis, being processed to generate a free toxin fragment which is translocated into the cytoplasm, and finally induction of cell death by catalytic inactivation of elongation factor 2. In phase-I testing BL22 was very active in chemoresistant hairy-cell leukemia (HCL), with 19 (61%) of 31 patients achieving complete remission (CR). The low blood counts (cytopenias) which are characteristic of HCL improved in all complete and partial responders. Dose-limiting toxicity in HCL was due to a reversible hemolytic uremic syndrome (HUS), observed only during cycles 2 or 3. Already under way are a phase-II trial in HCL and phase-I trials in chronic lymphocytic leukemia (CLL) and acute lymphocytic leukemia (ALL) administering BL22 in a modified protocol in an effort to prevent HUS.

PMID: 16997177

Once again (and to our chagrin), BL22 has not been as effective in CLL, perhaps because the target CD22 is not expressed as well on CLL cells.


Link: JCO Article (FREE full text link)

J Clin Oncol. 2005 Sep 20;23(27):6719-29. Epub 2005 Aug 1

Phase I trial of recombinant immunotoxin RFB4(dsFv)-PE38 (BL22) in patients with B-cell malignancies

Kreitman RJ, Squires DR, Stetler-Stevenson M, Noel P, FitzGerald DJ, Wilson WH, Pastan I.

Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bldg 5124b, 9000 Rockville Pike, Bethesda, MD 20892

PURPOSE: To conduct a phase I trial of recombinant immunotoxin BL22, an anti-CD22 Fv fragment fused to truncated Pseudomonas exotoxin.
PATIENTS AND METHODS: Forty-six pretreated patients with CD22+ non-Hodgkin's lymphoma (NHL; n = 4), chronic lymphocytic leukemia (CLL; n = 11), and hairy cell leukemia (HCL; n = 31) received 265 cycles at 3 to 50 microg/Kg every other day x 3 doses.
RESULTS: BL22 was active in HCL, with 19 complete remissions (CRs; 61%) and six partial responses (PRs; 19%) in 31 patients. Of 19 CRs, 11 were achieved after one cycle and eight after two to 14 cycles. All 25 responders benefited clinically with one cycle. The CR rate was 86% in patients enrolled at > or = 40 microg/Kg every other day x 3, and 41% at lower doses (P = .011). The median duration for CR was 36 months (range, 5 to 66 months), and eight patients remain in CR at 45 months (range, 29 to 66 months). Lower but significant activity occurred in CLL. Neutralizing antibodies occurred in 11 (24%) of 46 patients (all HCL). A reversible hemolytic uremic syndrome requiring plasmapheresis was observed in one patient with NHL during cycle 1 and in four patients with HCL during cycle 2 or 3. The maximum-tolerated dose (MTD) evaluated at cycle 1 was 40 microg/Kg IV. QOD x 3. The most common toxicities at 30 to 50 microg/Kg every other day x 3 included hypoalbuminemia, transaminase elevations, fatigue, and edema.
CONCLUSION: BL22 was well tolerated and highly effective in HCL, even after one cycle. Phase II testing is underway to define the efficacy with one cycle and to study safety when additional cycles are needed for optimal response.

PMID: 16061911


Perhaps we should consider this latest immunotoxin trial using CAT-8015 from the perspective of past history with similar drugs.

LMB-2 clinical trial was reviewed earlier on this website, just about three years ago on April 13, 2004 (LMB-2). Ontak clinical trial results were reviewed a couple of days later in a Topics Alert (Alert Number 13: Results of Phase -2 Ontak in Relapsed CLL Patients). Reading through what I wrote back then has been a salutary experience for me. Back then, I was a lot more bullish about the potential value of LMB-2 and Ontak in the treatment of CLL – my optimism of that period has not been matched by the progress of these drug in the past three years.

Much of the work on LMB-2 was also done at the NCI. The names of many of the same investigators show up on the LMB-2, BL22, and now CAT-8015 abstracts. I would like to think that three years later the science has improved and the new immunotoxin CAT-8015 will be better than its earlier cousins Ontak, BL22 and LMB-2. But it is important for us to learn from past disappointments. Progress of science is a slow process, and that is a reality we have to live with. Three years may seem like an eternity to patients facing therapy decisions but it is no more than a blink of the eye to people in the trenches dealing with funding, regulatory approvals, clinical trial recruitment and publication. Patients and researchers march to very different drum beats, my friends.

Immunotoxins face a few real hurdles that have yet to be conquered, and you can read about them in great detail in the review article cited below.

This is an early stage clinical trial, looking specifically to get information on the right dose to use with manageable toxicity. I am all for patients volunteering for clinical trials. How else are we ever going to get new and more effective therapies for our community? However, I am also very much of the opinion that patient participation should be a truly informed decision, not something on which they get “sold”. I have tried to describe the science involved in this article. Here is the other thing you should know. Heck, you should be able to recite this in your sleep by now. Phase I clinical trials are meant for understanding the science of how new drugs operate and what dosage to use in future trials. They are rarely meant to provide any therapeutic benefit to the participants in the trials. If you are going into this or any other Phase I trial with the hope of getting a miraculous cure for your CLL, my guess is you are being (a little) overly optimistic.


Link: AAPS Journal Article (FREE full text link)

AAPS Journal. 2006; 8(3): E532-E551. DOI:

Immunotoxins for Targeted Cancer Therapy

Robert J. Kreitman

Clinical Immunotherapy Section, Laboratory of Molecular Biology, Centers for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD

Accepted: June 14, 2006; Published: August 18, 2006

Immunotoxins are proteins that contain a toxin along with an antibody or growth factor that binds specifically to target cells. Nearly all protein toxins work by enzymatically inhibiting protein synthesis. For the immunotoxin to work, it must bind to and be internalized by the target cells, and the enzymatic fragment of the toxin must translocate to the cytosol. Once in the cytosol, 1 molecule is capable of killing a cell, making immunotoxins some of the most potent killing agents. Various plant and bacterial toxins have been genetically fused or chemically conjugated to ligands that bind to cancer cells. Among the most active clinically are those that bind to hematologic tumors. At present, only 1 agent, which contains human interleukin-2 and truncated diphtheria toxin, is approved for use in cutaneous T-cell lymphoma. Another, containing an anti-CD22 Fv and truncated Pseudomonas exotoxin, has induced complete remissions in a high proportion of cases of hairy-cell leukemia. Refinement of existing immunotoxins and development of new immunotoxins are underway to improve the treatment of cancer.



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