Sorry about not answering your question earlier.
In the United States, giving two injections to people over time will require an additional surgery and each surgical procedure will likely cost about $50,000. Two injections will cost $100,000. By the way, it is not so easy or cheap to do experiments in animals either. The manpower required for animal experiments is enormous. Each anesthesia and laminectomy, particularly in rats that have already been spinal-injured, will result in some mortality. Let me give an example of what it would take to undertake an experiment to compare the efficacy of one versus two cell injections into the spinal cord of rats with chronic spinal cord injury.
Suppose you are just comparing surgery with no injections, surgery with one injection of cells, and surgery with two injections of cells each separated by a month. Even with our NYU contusion model, which is the most precise and most efficient model in the field, you should do about 12 animals per treatment group to detect a 10% difference in locomotor score (assuming 1-2 mortalities per group). You have to have concurrent controls. So, the experiments would require 5 groups of 12 rats = 60 rats, i.e. control injury with no injection, control 1 injection, treatment 1 injection, control 2 injections, treatment 2 injections.
Imagine taking care of 60 rats for 14 weeks (6 weeks when they are "chronic", treat them, and then another 8 weeks to see the effects of the therapy). You have to squeeze their bladders every day. Let's say it takes you 5 minutes to inspect and squeeze each rat. This means that, if you are very efficient and champion rat squeezer, you can do 12 rats an hour and it will take you 5 hours to squeeze all 60 rats once a day without any breaks. You of course have to give them antibiotics and other treatments if you find that they have infections or neuropathic pain. You may have to squeeze some rats twice a day, if you fail the first time. This has to happen 7 days a week. Just bladder care of the rats alone will require two full-time experienced animal caretakers.
The surgery for spinal cord injury and the surgery to expose the spinal cord for injecting the cells will each take 2-3 hours per rat. So, 3-4 surgeries will take up a whole day. In a week, if you operate 8 hours a day for 5 days, you can operate on at most 20 rats. A good surgeon will require at least 3 weeks to operate on 60 rats to do the spinal cord injury. At 6 weeks and 10 weeks after injury, the surgeon will have to re-anesthetize the rats and go through 3 weeks of surgery to treat the rats with the first injection and the rats against with the second injection. To assess walking of the animals, you have to examine each animal once a week. Finally, you have to anesthetize the rat, perfused it, remove the spinal cords, post-fix the spinal cords, section and stain the spinal cords, photograph the sections with a microscope, and analyze the data. A good technician can perhaps analyze the spinal cord of two rats a week.
So, a team of 5 people are required for this study for 6 months, i.e. two animal care technicians, one surgeon, one histologist/pathologist, and one supervising investigator. By the way, the cost to do such an experiment will cost as much as $312,000, assuming 44% fringe benefit rates. If you get a grant from the NIH to do such an experiment, the cost will be $500,000, assuming 60% indirect cost rates. I attach a spreadsheet of a sample budget.
So, it is an expensive proposition to test and show that multiple injections are better than one. I agree with you that it needs to be done, however. One of the things that we are hoping to do in our clinical trial is to include patients who have already been in our trials and then enrolling them in a trial again for when their recovery comes to a plateau, to see if their improvements are boosted by a second or even a third transplant.