Intracellular Bacteria and Protozoa - Intracellular Delivery: Fundamentals and Applications - page 777

Plasmodium invades cells by pulling itself into the cell by an actin-myosin motor
that is present in their own cytoskeleton. Then a vacuola is formed by the host plasma
membrane. Parasites resist phagosome-lysosome fusion by exclusion of host-cell
integral membrane proteins that regulates the endocytic/phagocytic pathway from
these vacuola, whereas parasite's proteins are incorporated. This result in the forma-
tion of a unique PV enclosed by a semi-permeable membrane with channels or pores
with a size exclusion limit of <1,300-1,400 Da (Desai et al. 1993 ; Schwab et al.
1994 ; Desai and Rosenberg 1997 ; Werner-Meier and Entzeroth 1997 ). It is important
to note that following infection by Plasmodium ; membrane channels called 'new
permeability pathways' (NPPs) appear on host plasma membrane that allow the pas-
sage of solutes to the intracellular space (Biagini et al. 2005 ). Macromolecules such
as dextran, A protein and IgG2a (Pouvelle et al. 1991 ) and also latex nanoparticles
of 50-80 nm have access to parasite through these channels (Goodyer et al. 1997 ).
Additionally the PV membrane of Plasmodium extends into the host cell cyto-
plasm and forms a network of tubulovesicular membranes (TVM). The TVM acts
as a molecular sieve which forms a junction with the erythrocyte membrane which
allows import of specific nutrients such as adenosine, glutamate and orotic acid to
the parasite (Lauer et al. 1997 ).
The erythrocytic stages (trophozoites, schizontes and gametocytes) living inside
PV and the exoerythrocytic stages (intra-hepatocytes hypnozoites) present major
structural and phenomenological barriers that antimalarial drugs have to
overcome.

6.1 Conventional Treatments

Therapy depends on Plasmodium specie, severity of infection, previous exposition
to infection, clinic status of patient and area where the infection was acquired and
its drug-resistance status. Most drugs are active against the parasite forms in the
blood and include: chloroquine, atovaquone-proguanil, artemether-lumefantrine,
mefloquine, quinine, quinidine, doxycycline (used in combination with quinine),
clindamycin (used in combination with quinine) and artesunate (Santos-Magalhães
and Furtado Mosqueira 2010 ). The main drawbacks of conventional malaria che-
motherapy are the development of multiple drug resistance and the non-specific
targeting to cells free from intracellular parasites, resulting in high dose require-
ments and subsequent intolerable toxicity.
The four-aminoquinolone chloroquine phosphate was until recently the most
widely used anti-malarial. It is accumulated in the food vacuoles (pH 5.2-5.8) and
acts by inhibiting the hemozoin biocrystallization, thus facilitating an aggregation
of cytotoxic heme. The main advantages of chloroquine therapy are the fast action
in blood parasite stages, low toxicity, good bioavailability from oral dosage form,
water solubility, high volume of distribution in the body and low cost. The main
disadvantage is the widespread resistance due to a very long terminal elimination
half-time (1-2 months).