Proguanil

When the Japanese attack on Pearl Harbor started World War II in the Pacific, the US became very interested in antimalarial medications and funded a large joint US-UK program to find new non-toxic and easy to produce drugs of the type.{{cite journal | url=https://www.academia.edu/7066790 | title=War and Change in the Pharmaceutical Industry: A Comparative Study of Britain and France in the Twentieth Century | journal=Entreprises et Histoire | year=2004 | volume=36 | issue=2 | page=64 | vauthors = Quirke VM | doi=10.3917/eh.036.0064 }} It was joined by a team led by Frank Rose at the Medical Chemicals Section of Imperial Chemical Industries (later Pharmaceuticals Division, which ended up demerged into Zeneca) at Blackley, which earlier developed a way to manufacture mepacrine, an antimalarial made exclusively in Germany before the war.

Rose and his colleague Frank Curd decided to concentrate on pyrimidines as relatively simple to synthetise, even though the Advisory Panel recommended against that because most antimalarials by then were either quinolines or acridines. Checking prospective 2,4-diaminopyridine derivatives with a basic side chain and a benzenoid moiety one after another, they noticed a geometric pattern in the effective analogs and wondered if they could reproduce their interesting biologic activity with molecules even simpler, without the pyrimidine ring, and tried biguanides (then called diguanides) with which Rose was familiar due to his earlier sulphonamide research to great effect.{{cite journal | vauthors = Suckling CW, Langley BW | title = Francis Leslie Rose: 27 June 1909-3 March 1988 | journal = Biographical Memoirs of Fellows of the Royal Society | volume = 36 | issue = | pages = 491–524 (503-505) | date = 1990 | pmid = 11616178 | doi = 10.1098/rsbm.1990.0041| s2cid = 40302178 | url = https://royalsocietypublishing.org/doi/pdf/10.1098/rsbm.1990.0041 }} The drug was introduced by ICI in 1945.

Proguanil is used for the prevention and treatment of malaria in both adults and children, particularly in areas where chloroquine-resistant P. falciparum malaria has been reported. It is usually taken in combination with atovaquone, another antimalarial drug.{{Cite encyclopedia|url=https://medlineplus.gov/ency/article/000621.htm|title=Malaria | encyclopedia = MedlinePlus Medical Encyclopedia | publisher = U.S. National Library of Medicine |access-date=16 November 2016|url-status=live|archive-url= https://web.archive.org/web/20161117064956/https://medlineplus.gov/ency/article/000621.htm |archive-date=17 November 2016}}

It is also effective in the treatment of most other multi-drug resistant forms of P. falciparum; the success rate exceeds 93%.{{cite journal | vauthors = Boggild AK, Parise ME, Lewis LS, Kain KC | title = Atovaquone-proguanil: report from the CDC expert meeting on malaria chemoprophylaxis (II) | journal = The American Journal of Tropical Medicine and Hygiene | volume = 76 | issue = 2 | pages = 208–223 | date = February 2007 | pmid = 17297027 | doi = 10.4269/ajtmh.2007.76.208 | doi-access = free }}

Proguanil is generally well tolerated, and most people do not experience side effects. However, common side effects include abdominal pain, nausea, headache, and fever. Taking proguanil with food may lessen these side effects.{{Cite web|url=http://www.mayoclinic.org/drugs-supplements/atovaquone-and-proguanil-oral-route/side-effects/drg-20061484|title=Atovaquone And Proguanil (Oral Route) Side Effects | work = Mayo Clinic |access-date=8 November 2016|url-status=live|archive-url=https://web.archive.org/web/20161109090547/http://www.mayoclinic.org/drugs-supplements/atovaquone-and-proguanil-oral-route/side-effects/drg-20061484|archive-date=9 November 2016}} Proguanil should not be taken by people with severe renal impairment, pregnant women, or women who are breastfeeding children less than 5 kg.{{Cite web|url=https://www.cdc.gov/malaria/travelers/drugs.html|title=CDC - Malaria - Travelers - Choosing a Drug to Prevent Malaria | work = U.S. Centers for Disease Control and Prevention (CDC) |access-date=8 November 2016|url-status=live|archive-url=https://web.archive.org/web/20161113071230/http://www.cdc.gov/malaria/travelers/drugs.html|archive-date=13 November 2016}} There have also been reports of increased levels of liver enzymes, which may remain high for up to 4 weeks after completion of treatment.{{cite journal | vauthors = Looareesuwan S, Wilairatana P, Chalermarut K, Rattanapong Y, Canfield CJ, Hutchinson DB | title = Efficacy and safety of atovaquone/proguanil compared with mefloquine for treatment of acute Plasmodium falciparum malaria in Thailand | journal = The American Journal of Tropical Medicine and Hygiene | volume = 60 | issue = 4 | pages = 526–532 | date = April 1999 | pmid = 10348224 | doi = 10.4269/ajtmh.1999.60.526 | s2cid = 32365159 | doi-access = }}

When used alone, proguanil functions as a prodrug. Its active metabolite, cycloguanil, is an inhibitor of dihydrofolate reductase (DHFR).{{Cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/proguanil#section=Top|title= Proguanil {{!}} C11H16ClN5 | work = PubChem | publisher = U.S. National Library of Medine |access-date=13 November 2016|url-status=live|archive-url=https://web.archive.org/web/20161114165700/https://pubchem.ncbi.nlm.nih.gov/compound/proguanil#section=Top|archive-date=14 November 2016}} Although both mammals and parasites produce DHFR, cycloguanil's inhibitory activity is specific for parasitic DHFR. This enzyme is a critical component of the folic acid cycle. Inhibition of DHFR prevents the parasite from recycling dihydrofolate back to tetrahydrofolate (THF). THF is required for DNA synthesis, amino acid synthesis, and methylation; thus, DHFR inhibition shuts down these processes.{{cite journal | vauthors = Boggild AK, Parise ME, Lewis LS, Kain KC | title = Atovaquone-proguanil: report from the CDC expert meeting on malaria chemoprophylaxis (II) | journal = The American Journal of Tropical Medicine and Hygiene | volume = 76 | issue = 2 | pages = 208–223 | date = February 2007 | pmid = 17297027 | doi = 10.4269/ajtmh.2007.76.208 | doi-access = free }}

Proguanil displays synergism when used in combination with the antimalarial atovaquone. This mechanism of action differs from when proguanil was used as a singular agent. In this case, it is not thought to function as a DHFR inhibitor. The addition of proguanil has shown to reduce resistance to atovaquone and increase the ability of atovaquone to trigger a mitochondrial apoptotic cascade.{{cite journal | vauthors = Srivastava IK, Vaidya AB | title = A mechanism for the synergistic antimalarial action of atovaquone and proguanil | journal = Antimicrobial Agents and Chemotherapy | volume = 43 | issue = 6 | pages = 1334–1339 | date = June 1999 | pmid = 10348748 | pmc = 89274 | doi = 10.1128/AAC.43.6.1334 }} This is commonly referred to as "collapse of the mitochondrial membrane potential."{{cite journal | vauthors = Srivastava IK, Rottenberg H, Vaidya AB | title = Atovaquone, a broad spectrum antiparasitic drug, collapses mitochondrial membrane potential in a malarial parasite | journal = The Journal of Biological Chemistry | volume = 272 | issue = 7 | pages = 3961–3966 | date = February 1997 | pmid = 9020100 | doi = 10.1074/jbc.272.7.3961 | doi-access = free }} Proguanil lowers the effective concentration of atovaquone needed to increase permeability of the mitochondrial membrane.{{cite journal | vauthors = Thapar MM, Gupta S, Spindler C, Wernsdorfer WH, Björkman A | title = Pharmacodynamic interactions among atovaquone, proguanil and cycloguanil against Plasmodium falciparum in vitro | journal = Transactions of the Royal Society of Tropical Medicine and Hygiene | volume = 97 | issue = 3 | pages = 331–337 | date = May 2003 | pmid = 15228254 | doi = 10.1016/S0035-9203(03)90162-3 | doi-access = }}

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