4-N-, 4-S-, and 4-O-chloroquine analogues: influence of side chain length and quinolyl nitrogen pKa on activity vs chloroquine resistant malaria

Abstract

Using predictions from heme-quinoline antimalarial complex structures, previous modifications of chloroquine (CQ), and hypotheses for chloroquine resistance (CQR), we synthesize and assay CQ analogues that test structure-function principles. We vary side chain length for both monoethyl and diethyl 4-N CQ derivatives. We alter the pKa of the quinolyl N by introducing alkylthio or alkoxy substituents into the 4 position and vary side chain length for these analogues. We introduce an additional titratable amino group to the side chain of 4-O analogues with promising CQR strain selectivity and increase activity while retaining selectivity. We solve atomic resolution structures for complexes formed between representative 4-N, 4-S, and 4-O derivatives vs mu-oxo dimeric heme, measure binding constants for monomeric vs dimeric heme, and quantify hemozoin (Hz) formation inhibition in vitro. The data provide additional insight for the design of CQ analogues with improved activity vs CQR malaria.

Figure 1

A. Structures of drug – μ-oxo dimer complexes derived from distance geometry calculations using Fe(III)-drug (¹H) distance restraints from relaxation measurements. The drug molecules, on average, are approximately 3-4 Å above the plane of the porphyrin ring. Since the distance restraints are drawn from a single point (Fe(III)), the porphyrin plane’s rotational orientation is not unequivocally defined (see Fig 1B). Within the limitations imposed by assumptions made in these calculations and the accuracy of the data, no significant differences in how these drug molecules interact with the μ-oxo dimer are found. B. As in 1A, but top-down view. The relaxation rates of the alipathic protons are likewise enhanced by the addition of heme and as shown in these structures, the side chains do not extend away from Fe(III), but trace the perimeter of the porphyrin ring (see also references and 18).

Figure 1

A. Structures of drug – μ-oxo dimer complexes derived from distance geometry calculations using Fe(III)-drug (¹H) distance restraints from relaxation measurements. The drug molecules, on average, are approximately 3-4 Å above the plane of the porphyrin ring. Since the distance restraints are drawn from a single point (Fe(III)), the porphyrin plane’s rotational orientation is not unequivocally defined (see Fig 1B). Within the limitations imposed by assumptions made in these calculations and the accuracy of the data, no significant differences in how these drug molecules interact with the μ-oxo dimer are found. B. As in 1A, but top-down view. The relaxation rates of the alipathic protons are likewise enhanced by the addition of heme and as shown in these structures, the side chains do not extend away from Fe(III), but trace the perimeter of the porphyrin ring (see also references and 18).

Figure 2

A suggested structure for a drug – μ-oxo dimer complex, in which the drug has a branched side chain. One of the branches is placed along the perimeter of the porphyrin ring, as seen in Figure 1B and for previously solved CQ, QN, QD, and AQ structures,, while the other branch extends away from the ring. In this arrangement, it is possible that this terminal amino group then forms an a hydrogen bonding pair with the propionate side chain of heme. A minimal distance (> 4 methylenes between terminal amino and the branch point) for both maximal π – π interaction and hydrogen bonding is defined in this structure.

Scheme 1

Synthesis of chloroquine derivatives 1 – 10

Scheme 2

Synthesis of CQ – derived ethers

Scheme 3

Synthesis of CQ – derived sulfides

Scheme 4

Introduction of α,ω–diaminoalkoxy branched sidechains to 7-chloroquinoline