SP is the abbreviation of Sulphadoxine + Pyrimethamine, a combination drug used to treat malaria.

See this paper about it http://www.sti.ch/pdfs/(03)35-44.pdf
You can read:

As a result of the development of resistance, chloroquine
(CQ) has been replaced with the combination of *** sulphadoxine/
pyrimethamine (SP) *** as first line anti-malarial drug in several
African countries. However, resistance to this combination is
also spreading rapidly. The monitoring of this resistance plays
an important role in indicating when new policy changes should
take place.

Also in it:
Drug targets and drug resistance in malaria parasites
Molecular typing of drug resistance markers offers another
approach for measuring the extent of drug resistance. Resistance
to *** pyrimethamine and sulphadoxine (SP) *** has been associated
with point mutations at the genes encoding two enzymes
in the pathway of *** folic acid synthesis ***(DHFR and DHPS).

Another reference for "SP":
Effectiveness of intermittent preventive treatment with ... Intermittent preventive treatment (IPT) with *** sulphadoxine-pyrimethamine (SP) *** in the second and ... more likely to have received IPT, and iron and/or folic acid. ...
www.blackwell-synergy.com/links/ doi/10.1111/j.1365-3156.2004.01196.x/abs/

The two active components of the drug product works inhibiting the production of folic acid by Plasmodium, the parasite responsible of malaria. See a brief explanation in this drug information paper:
http://www.healthnetwork.com.au/search-display.php?cat=medic...

You can read about Pyrimethamine

Pyrimethamine

Pyrimethamine (Brand: Daraprim) *** works by inhibiting the parasite making folic acid, a critical vitamin for reproduction ***. *** In combination with an antibiotic, such as pyrimethamine plus sulphadoxine (Brand: Fansidar) ***, it offers better protection. These medications are usually prescribed in combination with chloroquine. The use of these agents has decreased, with doxycycline and mefloquine used as alternatives.




--------------------------------------------------
Note added at 4 hrs 55 mins (2004-08-17 07:30:39 GMT)
--------------------------------------------------

A last proof about the pharmacological \"nature\" of SP are the final words of the phrase. Elimination half-life is a parameter used in drug pharmacokinetics (the analysis of drug absorption, metabolism and elimination in the human body).