The relationship of physico-chemical properties and structure to the differential antiplasmodial activity of the cinchona alkaloids.
BACKGROUND
The 8-amino and 9-hydroxy substituents of antimalarial cinchona alkaloids have the erythro orientation while their inactive 9-epimers are threo. From the X-ray structures a 90 degrees difference in torsion angle between the N1-H1 and C9-O12 bonds in the two series is believed to be important. In order to kill the malaria parasite, alkaloids must cross the erythrocyte and parasite membranes to accumulate in the acid digestive vacuole where they prevent detoxication of haematin produced during haemoglobin breakdown.


METHODS
Ionization constants, octanol/water distribution and haematin interaction are examined for eight alkaloids to explain the influence of small structural differences on activity.


RESULTS
Erythro isomers have a high distribution ratio of 55:1 from plasma to the erythrocyte membrane, while for the more basic threo epimers this is only 4.5:1. This gives an increased transfer rate of the erythro drugs into the erythrocyte and thence into the parasite vacuole where their favourable conformation allows interaction with haematin, inhibiting its dimerization strongly (90 +/- 7%) and thereby killing the parasite. The threo compounds not only enter more slowly but are then severely restricted from binding to haematin by the gauche alignment of their N1-H1 and C9-O12 bonds. Confirmatory molecular models allowed measurement of angles and bond lengths and computation of the electronic spectrum of a quinine-haematin complex.


CONCLUSIONS
Differences in the antiplasmodial activity of the erythro and threo cinchona alkaloids may therefore be attributed to the cumulative effects of lipid/aqueous distribution ratio and drug-haematin interaction. Possible insights into the mechanism of chloroquine-resistance are discussed.
D000818:Animals; D000981:Antiprotozoal Agents / Q000737:chemistry* / Q000493:pharmacokinetics* / Q000494:pharmacology; D002738:Chloroquine / Q000493:pharmacokinetics / Q000494:pharmacology; D002930:Cinchona Alkaloids / Q000737:chemistry* / Q000493:pharmacokinetics* / Q000494:pharmacology; D015195:Drug Design; D004351:Drug Resistance / Q000502:physiology; D006420:Hemeproteins / Q000037:antagonists & inhibitors; D008958:Models, Molecular; D021261:Parasitic Sensitivity Tests; D010963:Plasmodium falciparum / Q000737:chemistry / Q000187:drug effects / Q000502:physiology; D021281:Quantitative Structure-Activity Relationship; D011803:Quinine / Q000031:analogs & derivatives / Q000737:chemistry / Q000493:pharmacokinetics / Q000494:pharmacology; D013237:Stereoisomerism
D000981:Antiprotozoal Agents; D002930:Cinchona Alkaloids; D006420:Hemeproteins; C020753:hemozoin; D002738:Chloroquine; D011803:Quinine