Improving on a good thing

You thought monoclonal antibody (MOAB) Rituxan was the greatest thing since sliced bread? Well, get ready for “bispecific antibodies” that may soon prove to do the job a whole lot better.  But before we get to the juicy details, you have to do a bit of reading - promise it won’r hurt even a little bit!

Here is a simple (some may say way over simplified) picture of how conventional monoclonal antibodies look.  Rituxan, Campath, Arzerra are all  “Y” shaped molecules similar to the  picture on the left.  The tips of the two arms of the “Y” are the pincers with which the antibody molecule is able to grab on to its target.  In Rituxan’s case, the target is the CD20 marker present on all mature B-cells, which includes CLL cells.  If you picture Rituxan molecule as a weird kind of lobster, the two pincers of the beast are what allow it to catch its prey.  Once the hapless CLL cell is festooned with a bunch of these “lobsters”, it is as if there is a huge “kill me” sign painted on the cancer cell and it is quickly killed by the rest of the immune system.  When it comes to killing tagged cells, T-cells are the ones that do most of the heavy lifting.

So, why does Rituxan not work all that well as a single agent in CLL?  The reason is that CLL cells do not express a whole lot of CD20 markers on their surface, which means not too many Rituxan molecules can tag the cancer cell, which in turn means the CLL cell does not always attract the attention of killer T-cells.  The “kill me” sign is painted on in very small letters, as it were.  One of the reasons why it is suggested that ofatumumab (Arzerra) works better than Rituxan is that it is thought to hang on a lot better to the CLL cell without letting go - think of it as better Velcro making the drug molecule stick better to the cancer cell.  Even then, there is no guarantee that the tagged CLL cell will automatically attract killer T-cells.  This is particularly so in the case of previously treated patients, since they may not have a whole lot of able and willing T-cells patrolling the body any more.

Dating service of the fatal kind

Here is the concept in a nutshell.  Won’t it be nice if we can introduce the killer T-cells to the cancerous CLL cells, up close and personal? Mr. T-cell, please meet Ms. CLL cell.  Ms. Cancerous CLL cell, please meet your nemesis, the ever so handsome Mr. Killer T-cell.  If we can bring the two cells close together, predator and prey, chances are much better that we will get the desired “chemistry”, and the T-cell will kill the CLL cell.

That is pretty much what Micromet’s “BiTE” (Bispecific T-cell Engager) attempted to do - and the latest results suggest they have done a pretty good job of it.

  • For starters, Micromet focused on CD19 as the target marker on B-cells.  Unlike CD20 (which is expressed to greater or lesser degree depending on the specific nature of the B-cell), all B-cells must express CD19 in copious amounts, no exceptions allowed. Without CD19, a B-cell cannot be a B-cell, it is a classic identifier of all B-cells starting from very young immature B-cells all the way to mature B-cells.
  • Next, they looked for a classic marker on T-cells that is involved in activating the T-cell and making it a fearless killer cell.  This happens to be the CD3 marker.
  • Rather than having a humongously large “Y” shaped molecule such as Rituxan, Campath etc, Micromet managed to get just the tips of the “Y”, the very pincers of the lobster as it were.  They attached one pincer that targets CD19 with another pincer that targets CD3 - connected the two pincer tips with a short peptide chain.  The picture below may make the concept easier to visualize

Since MT103 combines just the business ends, not the whole big “Y” shaped molecules that are conventional monoclonal antibodies, what we have here is a short rod shaped molecule, very tiny compared in size to a Rituxan molecule.  One end of MT103 tags CD3 marker on T-cells, the other end of the rod tags CD19 marker  on B-cells. Very simple, very elegant, very compact

As it happens, this compactness is exactly what was needed. It seems the T-cell has to be sufficiently close to the target cell before it can “recognize” it, and then kill it. Since Micromet’s MT103 molecule is a very short rod shaped molecule, it is possible to bring the killer T-cell into close proximity of the victim B-cell.

Voila! Match made in hell. The CD3 hook latches on to a T-cell, and as soon as a B-cell happens by, the other end of the rod with the CD19 hook latches on to that. Or vice-versa. We now have a target b-cell and a killing T-cell locked together in a tight and fatal embrace. No amicable divorce is possible here, this union ends when the B-cell is killed. The T-cell moves on to find its next victim.

If you type “bispecific antibodies” into PubMed search engine, you will get literally thousands of hits. They are a little tough to digest, so I thought I would do you guys a favor and give you a comic strip version of it.  You can get a more detailed explanation and glitzy pictures of how the technology works by visiting Micromet’s website.

Below is an abstract I used in reviewing this technology back in 2003 (my, where have all the years in between gone?). I told you I have been following this technology for a long time!  This early lab study using cell lines showed that the bispecific antibody approach was far more effective in cell kill.  Under identical lab conditions, BiTe technology was 100,000 fold more effective than Rituxan!

Int J Cancer 2002 Aug 20;100(6):690-7

Extremely potent, rapid and costimulation-independent cytotoxic T-cell response against lymphoma cells catalyzed by a single-chain bispecific antibody.

Dreier T, Lorenczewski G, Brandl C, Hoffmann P, Syring U, Hanakam F, Kufer P, Riethmuller G, Bargou R, Baeuerle PA.

Micromet AG, Am Klopferspitz 19, 82152 Martinsried, Germany.

A recent study reported on an anti-CD19/anti-CD3 single-chain bispecific antibody (bscCD19xCD3) exhibiting high activity against human B lymphoma cell lines (Loffler et al., Blood 2000;95:2098-103). In the present study, we have explored in detail the in vitro efficacy, T-cell donor variability, binding characteristics, specificity, kinetics and interleukin-2 (IL-2) dependence of bscCD19xCD3. We found that a majority of human donor T cells tested (n = 86) gave half-maximal B-lymphoma cell lysis (ED(50)) within a range of 10-50 pg/ml bscCD19xCD3, corresponding to sub-picomolar concentrations of the bispecific antibody. Under identical experimental conditions, the anti-CD20 monoclonal antibody rituximab had an at least 100,000-fold lower in vitro efficacy. The extreme potency of bscCD19xCD3 was in sharp contrast to the relatively low affinity of the anti-CD3 and anti-CD19 single-chain Fv portions in K (D) ranges of 10(-7) and 10(-9) M, respectively. Cell lysis by bscCD19xCD3 was predominantly mediated by the population of CD8/CD45RO-positive T cells. Both immortalized CD4- and CD8-positive human T-cell clones were highly active effector cells as well. Cell lysis by bscCD19xCD3 was rapid and specific. The respective parental monoclonal antibodies inhibited cell lysis and CD19-negative cells were not harmed by T cells in the presence of high amounts of bscCD19xCD3. The potent T-cell stimulus IL-2 could not markedly augment the activity of bscCD19xCD3-stimulated T cells. In conclusion, bscCD19xCD3 could redirect unstimulated cytotoxic T cells against CD19-positive cells in an unexpectedly potent, rapid and specific fashion.

Micromet has progressed a long way from lab cell line studies to actual clinical trials in live patients.  Their recent press release reporting the results of the clinical trial in ALL (acute lymphoblastic leukemia) caused a flurry of excitement.  So much so that the stock was discussed on “Mad Money” program by Jim Cramer ! (I cannot give you financial advice or stock pick tips, any more than I can give you medical advice, please remember that.  I personally own zero stocks in Micromet).

Blinatumomab” is the formal name of Micromet’s MT103 BiTe technology.  There is logic behind these outlandish names of drug molecules, but that is not really here nor there.  Just don’t let the names intimidate you.  I will continue using “BiTE” and MT103 since they are a whole lot simpler to type.

Blinatumomab Achieved 75% Complete Remission In Acute Relapsing Lymphoblastic Leukemia Patients

10 Jun 2011

75% of acute lymphoblastic leukemia patients who had relapsed after standard therapy achieved complete remission with blinatumomab, a medication designed to harness T cells that destroy cancer cells. Blinatumomab makers, Micromet says the drug is “the most advanced of a new class of agents called BiTE® antibodies.”

According to interim results of a Phase II single-arm trial, 9 out of 12 (75%) patients on blinatumomab went into complete remission, or complete remission with partial recovery of blood counts. They achieved a complete molecular response, or had no trace of leukemic cells in their bone marrow. Four patients with genetic abnormalities usually linked to poorer prognoses all achieved complete remission with partial recovery of blood counts.

Chair of the study, Professor Max Topp, Department of Internal Medicine II, University of Wuerzburg, said: “Relapsed/refractory acute lymphoblastic leukemia is a difficult to treat disease that has seen no meaningful improvement in decades. To date blinatumomab has shown an impressive and unprecedented level of efficacy in a patient population with limited therapeutic options.”

The most common negative side effects were peripheral edema, fatigue and fever. Of the twelve patients, two had to have their treatment interrupted because of fully reversible and manageable CNS (central nervous system) events. One developed cytokine release syndrome, which was dealt with by modifying dosage and administering pre-treatment concomitant medication, after which there were no cases of cytokine release syndrome.

Professor Topp said: “These results are particularly striking relative to the fact that the majority of enrolled patients had characteristics typically associated with a dismal outlook.” Standard therapy for those with relapsed/refractory acute lymphoblastic leukemia consists of combination chemotherapy medications, which often fail to achieve remission.

For over three decades no new medication has been approved for relapsing patients. Doctors’ options to improve patients’ long-term outcomes have been very limited. Current therapies achieve remission rates of between 17% and 45%. Standard chemotherapy typically has a mortality rate of up to 23%. On average, only 7% of acute lymphoblastic leukemia patients survive for at least five years after a relapse.

In this study, patients were given blinatumomab as a continuous infusion for 28 days, followed by two weeks with no treatment. Those who achieve complete remission (CR) or CR with partial recovery of blood counts (CRh) within the first two cycles of treatment may receive consolidation with either 3 more cycles of blinatumomab or allogeneic HSCT.

Why is this development of interest to us, CLL patients? for starters, ALL too is a B-cell cancer, just like CLL.  The same CD19/CD3 bispecific antibody would likely work in CLL too.  Micromet chose to do its early clinical trials with ALL because this acute cancer progresses fast, much more fatal quickly and therefore it is easier for them to showcase what their drug can do.  There are some disadvantages to having a “good cancer” like CLL, and in this case the disadvantage is that proving the value of a new drug takes much longer.  Drug companies are not known for their patience, they want to make money NOW.  But once the concept is proven and well established, there is no doubt in my mind that CLL will be in their cross-hairs.  The results they highlight here for ALL are pretty impressive, 75% complete and molecular responses in a very tough to treat bunch of patients who would otherwise be looking at significantly poorer response rates (17% - 45%) with conventional therapies.

Micromet has an open clinical trial using MT103 in NHL - and they include SLL patients in the definition of non-Hodgkin’s lymphoma. Unfortunately, the only centers where this clinical trial is available are in Germany.  But if any of our German members have SLL and fit the inclusion criteria, this may be a very good clinical trial to consider.  Please refer to the clinicaltrials.gov citation to get the full list of inclusion criteria, contact information etc.

If you are so inclined, here are a couple of much earlier articles I wrote on the subject of  new developments such as BiTe technology.

Is this how you spell C-U-R-E ?

Antibodies under development

I

Why target CD19? Like CD20 (targeted by Rituxan), CD19 is a marker expressed by B-cells. Unlike CD20, however, the CD19 marker is present on all developmental stages of B-cells and not just mature B cells. It is not, however, present on stem cells or plasma cells and is not expressed by T-cells, NK cells, stem cells, plasma cells or any other kind of cell line.  In terms of anti-CD19 therapy of the type we were discussing in my article (MT103 from Micromet), the antibody will target pre-B-cells as well as mature B-cells. In comparison to Rituxan therapy, where the target marker CD20 is expressed only on mature B-cells, the depletion of B-cell population by using anti-CD19 is likely to be more profound and deep. Thankfully, the stem-cells are not involved, otherwise CD19 would not be a safe or valid target for attack. And the advantage is that all mature B-cells express it, not just some of them. It (CD 19) is an essential feature of a B-cell being a B-cell, it cannot survive without it. Aha!! Problem number one above is solved. Unlike Rituxan therapy, where low levels of CD20 expression was a concern, no such problems here. And the action is similar to that of Rituxan, limited to only B-cells, since both CD20 and CD19 are expressed only by B-cells. No worries of across-the-board cell kill of all sorts of immune system cell lines, as in the case of Campath, for example. No worries about heavy duty immune suppression leaving you vulnerable to potential opportunistic infections.
As we described above, when compared to Rituxan, MT103 is much smaller in size. That smaller size and its rod-like shape means it can get into places that “fat lady Rituxan” could not, such as the middle of those large lymph nodes strategically placed at the sides of your neck, guaranteed to make you look like an overweight chipmunk.
One of the possible downsides is that MT103 does not hang around as long in the body, so we have to learn how to calculate the dosage and frequency of administration take that into account, get maximum bang for the buck.
So far we have discussed the CD19 end of the rod. How about the other end of the rod, the one with the pincer tip for CD3 marker? Well, this marker is expressed by mature T-cells, and when this marker is tagged, the T-cell becomes a “CTL”, a cytotoxic T-lymphocyte. No more worries about inefficient ADCC, and anergic T-cells sitting on the side lines like lazy bums, doing precious little. The targeted B-cell is hooked by the CD19 tip, and the killer T-cell is hooked by the CD3 tip. The two ends of the MT103 rod locks the target and effector cell together in a not-so-loving embrace, a match made in hell. (I rather liked the title of my previous article on this subject, “Fatal attraction of the cancer kind”. P.C thought it was a bit too cheesy, but heck, what does he know).
With ADCC going like gang busters, who cares about complement dependent pathways for cell-kill? CDC becomes less of an issue, as also the level of complement inhibitory proteins such as CD55 and CD59. This also reduces likelihood of developing resistance to multiple rounds of MT103, just in case you happen to need a mop up job a couple of years later.
With very effective cell-kill locked into place, so to speak, the dosages required are much less than that of Rituxan, to the tune of 100,000 fold less.
The manufacturers show that the effectiveness of the bscCD3XCD19 depends on the number of T-cells available, but that MT103 does its job so well and efficiently that it needs far fewer T-cells per B-cell that has to be killed. I tend to agree, but all the same, I would like a bit more work on that end of the equation. How about people in the other kind of watch and wait, who have been round the block a few more times than they care to remember? People with potentially depleted numbers or heavily compromised T-cells? Will the number of T-cells be enough to do the job in that case, and will the tagging (“ligation”) of the CD3 marker on the T-cells be enough to get them out of their lethargy to become lean and mean killing machines? Me, I like to get extra insurance. This is where the second of the two technologies comes in. But, before I forget, here is the website address of Micromet — do browse around - Micromet’s BiTE technology.