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In humans, what benefit do intraperitoneal (IP) injections(old/cheap rabies vaccines, or cancer related injections) offer versus traditional intramuscular injections?

For example, where I live, the rabies are not administered right away after a bite because of the associated discomfort(cheap IP injections) or cost(newer intramuscular injections) they represent versus the low probability of a dog having rabies and the long incubation period(instead, the offending dog is observed carefully for 10 days to decide, or the vaccines are administered in the fourth day if not found). So, since speed of action doesn't seem to be an important factor, what are the benefits of IP injections?

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    $\begingroup$ Likely has to do with how the injection will drain into the lymphatic system. cancerres.aacrjournals.org/content/24/10/1700.full.pdf and ncbi.nlm.nih.gov/pmc/articles/PMC2515477 $\endgroup$ – AMR Dec 17 '15 at 3:15
  • $\begingroup$ @AMR Interesting, mainly because in the cancer case, it seems that the main purpose seems to be that the fluid flows through the same channel as tumoral agents. $\endgroup$ – chubakueno Dec 17 '15 at 4:31
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    $\begingroup$ Remember that the adaptive immune system resides in the lymph, waiting to be activated... Though the rabies vaccine has been around longer than we have known about the intricacies of the lymphatic immunity. Pasteur invented the Rabies vaccine around 1885 and the lymph system role wasn't elucidated until many decades later. en.wikipedia.org/wiki/Timeline_of_immunology Maybe they just got lucky, they saw it worked, and well "if it ain't broke, don't fix it." $\endgroup$ – AMR Dec 17 '15 at 4:57
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    $\begingroup$ This might have some answers to your question. ncbi.nlm.nih.gov/pubmed/20156151 $\endgroup$ – AMR Dec 17 '15 at 5:05
  • $\begingroup$ @chubakueno we almost never give anything intraperitoneal in humans (I've never given an i.p. injection to a human) - this is done for rodents, dogs and other smaller animals. I see the article above from AMR - though that is for intraperitoneal chemotherapy which is given through a special surgically placed catheter (not injected) - what injection are you talking about specifically that you think is i.p.? $\endgroup$ – Vance L Albaugh Dec 18 '15 at 4:26
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IP injections are used for a variety of reasons:

  1. The peritoneum provides a large surface area for absorption of drug (compared with intramuscular (IM) or subcutaneous); can thus inject a larger fluid volume.

  2. Easier to inject that intravenous.

  3. For some chemotherapy, such as ovarian cancer, IP injection is used to attempt to localize the drug near the tumor (it's through a catheter, rather than with a syringe and needle). Essentially the goal is to provide chemotherapy to the abdominal region where cancer has spread rather than systemically.

It is commonly done in animals, such as mice, because it is difficult to find a vein in small creatures.

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    $\begingroup$ Nice answer +1. An additional point is that s.c. and i.m. only allow for small volumes to be injected, even in large animalia like humans. Plus, i.v. may result in a surge of drug in the body. I.p. allows for injection of large volumes, even in small animals, with a slowly-sloping absorption curve which may, or may not, be beneficial. $\endgroup$ – AliceD Dec 18 '15 at 11:52
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    $\begingroup$ The reason IP injections are common in mice is not for lack of veins, it's for convenience' sake. The tail vein, for example, is fine for injections, but it's so narrow that injections need to be in very small volumes and at very low rates to avoid blowing out the vein. The vein also needs to be expanded by bathing the animal in a heat lamp while restrained, all of which takes time and equipment, and is stressful for the animal. IP injections are easy: scruff, tip, inject, pull out, back in cage. $\endgroup$ – MattDMo Dec 19 '15 at 4:54
  • $\begingroup$ @MattDMo Completely agree, IP is much easier and less stressful on animal and researcher! $\endgroup$ – Minnow Dec 19 '15 at 4:56
  • $\begingroup$ @chubakueno it is important to realize that in humans we do not give routine injections intraperitonally - that only happens in animals. In fact, we intentionally do not insert needles into the peritoneal cavity for fear of potentially damaging intra-abdominal structures. However, if a patient has a peritoneal dialysis catheter or other intraperitonal access, we will administer things like antibiotics for a peritoneal infection or chemotherapy- but this is not for routine administration. $\endgroup$ – Vance L Albaugh Dec 29 '15 at 4:12

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