Stem cell harvest
- Methods for harvesting stem cells
- In-vivo purging before the harvest
- Options for poor mobilizers
- Detailed explanation of the harvest procedure
There are two methods for collecting enough Stem Cells for a transplant. Both methods apply equally to autologous or allogeneic donations.
Originally the collection was done by putting the donor under a general anaesthetic, and using a series of needles to extract bone marrow directly from the large bones, usually the pelvis. This was a relatively simple procedure but still has the same risks as any procedure which uses a general anaesthetic. The donor would feel no pain during the procedure, but would usually experience moderate pain for a few days after. This is the procedure they are referring to when you hear the term "Bone Marrow" harvest. (Harvest referring to the product collected from the donor)
The second type of harvest is the Peripheral Blood Stem Cell harvest. Bone Marrow collections have largely been abandoned in favour of this safer and better procedure. In a Peripheral Blood Stem Cell harvest, the donor is seated comfortably in an easy chair. An IV is placed into each arm. Blood is drawn out of one arm, sent to the aphaeresis machine where the CD34+ stem cells are separated, the the remaining blood is sent back into the IV in the other arm. In some cases a central venous catheter will be put in place if the patient does not appear to have veins capable of accepting the IV needles.
Peripheral Blood harvests have several advantages. The biggest and most important advantage is that the patient recovers faster, and with fewer major complications, when they receive a peripheral blood stem cell transplant, than if they receive a bone marrow transplant. Obviously anything that offers a survival and recovery benefit is of prime importance. Some other advantages are:
- No general anaesthetic is required for the donor
- The procedure can be repeated if enough are not collected the first time
- There is little or no pain to the donor during the collection procedure, although the Neupogen to mobilize the stem cells usually causes mild to severe bone pain as the marrow goes into overdrive producing more blood and stem cells. This bone pain can be quite severe for some, but is usually managed quite well with Tylenol or other OTC pain medications for most.
- There is a lower risk of collecting cancerous cells in an autologous harvest.
Because of these advantages, bone marrow harvests are not used very often. They are still used when the donor cannot harvest enough peripheral blood stem cells.
Increasingly researchers are looking for ways to ensure that the stem cell harvest in an autologous transplant is free from lymphoma cells. This contamination of the harvest is believed to contribute to relapses after SCT. The most common way to "purge" the harvest that is common use today is to use Rituxan as part of the mobilizing chemotherapy regimen. Rituxan readily attaches to any CD20+ cells in the bloodstream and helps ensure no lymphoma cells are included in the harvest. Below are some studies about using Rituxan purging (P.S. This webmaster had a Rituxan purged autologous SCT in 2002 and as of 2012 I remain in complete remission)
Prolonged Clinical and Molecular Remissions Following High-Dose Therapy/Autologous Stem Cell Transplantation (HDT/ASCT) and Rituximab and Interferon-alpha Maintenance for Relapsed High-Risk Follicular Lymphoma
Some patients are not able to harvest enough stem cells for a stem cell transplant. But this is not the end of the world. There are additional options to try which are often successful when standard harvesting has failed.
Plerixafor (brand name Mozobil in the USA) has shown great results and often results is significantly larger harvests in fewer sessions. Here are a couple of studies about Plerixafor/Mozobil (previously called AMD3100)
A Phase III, Multicenter, Randomized, Double-Blind, Placebo
Controlled, Comparative Trial of AMD3100 (Plerixafor)+G-CSF
vs. Placebo+G-CSF in Non-Hodgkins Lymphoma (NHL) Patients
for Autologous Hematopoietic Stem Cell (aHSC)
Another recent study shows that Paclitaxel is also effective at mobilizing poor mobilizers. You may be familiar with Paclitaxel from breast cancer and sometimes used in NHL, but it increases stem cell harvests. Here is a recent study.
Let's take a closer look at the peripheral blood stem cell harvest procedure. Normally there are not very many stem cells circulating in the blood. Certainly not anywhere near enough to collect. Therefore steps must be taken to encourage the body to push huge quantities of stem cells out of the bone marrow and into the blood where they can be collected. There are two ways to do this.
Give large doses of chemotherapy. When you do this it kills off large quantities of the white blood, so the bone marrow is forced to go into overdrive to replace it. Since there is not enough room in your bones to hold all the extra blood production, the bone marrow forces large quantities of stem cells out of the marrow and into the blood where they can mature quickly.
Give the donor a white blood growth factor called G-CSF which stands for Granulocyte Colony Stimulating Factor. The generic name for this drug is Filgrastim and the brand name is Neupogen. This drug encourages stem cells to mature very quickly into Neutrophils (a type of white blood cell). Because it causes such a rapid increase in neutrophils, the bone marrow is again forced to push the stem cells out of the marrow and into the blood to make room for all the new cells.
There is another drug which is helping the body to force even more stem cells into the circulating blood than Neupogen. It is called Plerixafor (brand name Mozobil). Either Plerixafor alone or in combination with Neupogen is successfully mobilizing stem cells in patients who previously failed. You can read more about Mozobil by going to Genzyme's website
In the case of an autologous transplant, where the patient is their own donor, they will use both methods at the same time. Not only does the chemotherapy help to reduce their cancer, but it also forces large numbers of stem cells into the blood. The Neupogen adds to this effect and makes successful harvest much more likely.
In the case of an allogeneic transplant, where a healthy person is the donor, you can't use the chemotherapy method because no healthy person is going to take toxic chemotherapy just to increase the stem cell collection. For allogeneic transplants only the Neupogen (with or without Plerixafor) is used since it is entirely safe.
Timing is critical for this procedure. Once those stem
cells have been forced into the blood, they want to mature
into neutrophils as quickly as possible. The whole key to
the procedure is to collect them before they do that, and
while they are still uncommitted stem cells. Usually it is
somewhere around 7 days after the chemotherapy and / or
Neupogen when the maximum number of stem cells will be
circulating. In many cases the donor will be tested daily to
find the optimum day. The testing is a simple blood test.
The harvest procedure is generally painless and quite easy for the donor. That of course not counting the pre-harvest bone pain as mentioned above due to the Neupogen.
On the day of the harvest the donor goes to the pheresis room and has a seat in a comfy chair or bed if required. An IV will be placed in both arms, although some donors will have a central venous catheter installed prior to the procedure to make it easier. As the blood passes through the pheresis machine the stem cells are separated, and removed, and the remaining blood is sent back to the donor. This procedure has a high probability of removing too much calcium from the blood as well. If you notice your lips tingling and going numb you know your calcium levels have fallen too much. Most people will simply eat a Tums or other calcium supplement at this point. As a precaution some centres will add calcium to the blood before it is returned to the patient to prevent this. In any case, if left untreated it could cause some serious reactions so every donor should be aware of this and watching for signs of tingling. Most collection centres try to make the donor as comfortable as possible by providing comfortable chairs, music, TV, or other items of comfort. It is also quite possible the donor will receive a bag of platelets after the collection because the collection procedure also removes a large number of platelets from the blood. This is most common when the donor is also the patient.
Unfortunately for the donor it is very important to keep the arms very still during this procedure (unless they have a central venous catheter). The collection machine is extremely sensitive to changes in the flow of blood and it will stop if the flow changes even a small amount. Small arm movements usually cause the machine to stop which simply makes the collection take longer.
If enough stem cells are not collected in the first attempt the donor will be asked to come back again, perhaps even a third or fourth time depending on circumstances. If they still can't collect enough stem cells, then that is where they may resort to an old fashioned bone marrow harvest.
In order to proceed with a Stem Cell transplant enough stem cells must be collected. The collection is measured in millions of stem cells per kilogram of the patients weight. It is always quoted that way and never as the total number of stem cells collected. This is because patient size makes a huge difference in the total number needed. In general the requirements are:
- Minimum required to perform a transplant: 2 million per kilogram
- Optimum preferred: 5 million per kilogram
- Bonus collection: 10 million per kilogram
Research shows while 2 million is sufficient, the patient will recover faster, and with a lower probability of grade 3 or 4 neutropenia if 5 million are collected. Research also shows that collecting 10 million does not really offer any recovery benefit over 5 million, but from the patients point of view, it does give an enormous psychological boost. There has been some recent evidence however that shows collecting more than 5 million may reduce the risk of later infections.
Some centres will do a transplant with less than 2 million units where the advantage to the patient outweighs the risk of serious complications due to slower recovery and engraftment. Generally this will be for patients with few other options.