Table 1: Anti-protozoal drugs that exhibit resistance

Drug	Resistance observed in	Mechanism of resistance
4-Aminoquinolines
Chloroquine ¹⁸	P. falciparum	A mutation in gene thus minimal drug accumulation
Amodiaquine ¹⁹		A gene (PfCRT, PfMDR1) that encodes a protein that helps API pass through the parasite membrane undergoes multiple mutations.
Piperaquine ²⁰		Multiple genetic mutations (PfCRT, Pfpm2, Pfpm3)
8-Aminoquinoline
Primaquine²¹	P. falciparum P. vivax	MDR1 gene mutation
Quinoline methanol
Mefloquine ²²	P. falciparum P. vivax	PfMDR1 gene that codes a protein engaged in transport of API across cell membrane of parasite
Cinchona alkaloids
Quinine ^{23 ,24}	Plasmodium spp.,	Genetic mutations
Quinidine²⁵	Plasmodium spp.,	Genetic mutations
Biguanides
Proguanil ²⁶	P. falciparum	DHFR gene mutations
Chlorproguanil²⁶	P. falciparum	DHFR gene mutations
Diaminopyrimidine
Pyrimethamine ²⁷	Plasmodium spp., Toxoplasma dondii	DHFR gene mutations
Sulfonamide and sulfone
Sulfadoxine ²⁸	Plasmodium spp.,	Gene mutation
Dapsone ²⁹		DHFR gene mutations
Sulfamethopyrazine		DHFR gene mutations
Naphthoquinone
Atovaquone	Plasmodium spp., Toxoplasma dondii	Cytochrome b gene mutation impeding the mitochondrial transport chain
Sesquiterpene lactones
Artemisinin and derivatives	P. falciparum	Delayed parasite clearance owing to propeller domain mutations
Pentavalent antimonial entities
Sodium stibogluconate ³⁰	L. donovani	Minimal drug uptake, elevated efflux, and thiol-based detoxification
Alkyl-phosphocholine derivative
Miltefosin ³¹	L. donovani	Mutations in transporter genes impair drug intake, efflux pumps, and drug sequestration
Nitroimidazole derivatives
Metronidazole ³²	G. lamblia T. vaginalis	Diminished drug activation owing to downregulation of pyruvate: ferredoxin oxidoreductase
Tinidazole ³³	Trichomonas vaginalis	Impaired drug activation and minimal drug uptake
Benznidazole ³⁴	Trypanosoma cruzi	Drives down nitro-reductase activity
Fexinidazole ³⁵	Trypanosoma brucei	Drives down nitro-reductase activity
Antibiotics
Paromomycin ³⁶	L. donovani	Ribosomal mutation and alterations in the drug intake pathway
Thiazolide
Nitazoxanide ³⁷	Cryptosporidium parvum	Probably the metabolic bypass pathway
Acridine
Quinacrine ³⁸	G. lamblia	Metabolic inactivation, in addition to altered intake
Diamines
Pentamidine ³⁹	Trypanosoma brucei, Leishmania	Mutation of TbAT1 gene
Suramin ⁴⁰	Trypanosoma brucei	Drives down suramin uptake receptor

Narrow therapeutic window:

It is a significant impediment to the efficacy of antiprotozoal drugs.⁴¹ It is acknowledged as tiny difference between effective dose and toxic dose of drug, making it tough to achieve optimal treatment outcomes without endangering the patient.

Table 2: Anti-protozoal drugs with narrow therapeutic windows

Drug	Mechanism	Adverse reactions
Chloroquine	Accumulate substantially in tissues	Gastrointestinal tract (GIT), skin adverse reactions, rare instances of severe allergic responses and renal failure, retinopathy, cardiac toxicity with prolonged and high dose usage ^{42, 43}
Primaquine	triggers oxidative stress inside cells, impedes RBC, thus hemolysis, necessitates glucose-6-phosphate dehydrogenase (G6PD) screening	Hemolysis, GIT unease, ⁴⁴
Melarsoprol⁴⁵	Host tissue toxicity	Extremely toxic; causes relative encephalopathy with high mortality, neurological impairment, and hypersensitivity
Nifurtimox ⁴⁶	Generates reactive oxygen species that impede host tissues	GI and CNS toxicity at therapeutic doses
Emetin ⁴⁷	Affects human ribosomes, thus the host tissue	Cardiotoxicity, myopathy, hypotension
Fexinidazole⁴⁸	Human African trypanosomiasis (T.b. gambiense)	CNS side effects include headache, nausea, insomnia, and hepatotoxicity with prolonged use
Liposomal Amphotericin B ⁴⁹	Impedes protein synthesis	Renal toxicity, infusion-related reactions
Paromomycin ⁵⁰	Impedes protein synthesis	Ototoxicity and nephrotoxicity
Quinacrine⁵¹	CNS toxicity	Skin discoloration, psychosis, GI upset
Pentamidine	Systemic toxicity and long tissue retention	Risk of nephrotoxicity, hepatotoxicity, arrhythmia, hypoglycemia
Suramin	Highly protein-bound and long half-life	Severe hypersensitivity reactions, nephrotoxicity, and adrenal insufficiency

Stage-specific activity:

This specifies a drug's potential to specifically target and eradicate the parasite at particular phases of its life cycle. For instance, certain drugs might be effective better against the blood stage of malaria than the liver stage.⁵² Drug's mechanism of action (MOA), parasite's life cycle and stage-specific vulnerabilities, drug's pharmacokinetic properties, such as its absorption, distribution, and elimination, host's immune response, and its impact on the parasite are pivotal factors that contribute to the stage-specific activity of anti-protozoal drugs.

Table 3: Anti-protozoal drugs exhibiting stage-specific activity

Drug	Target site	Limitations
Chloroquine	Erythrocytic stage of Plasmodium spp.⁵³	Ineffective against liver and gametocyte stages of P. falciparum
Primaquine	Dormant hypnozoite, mature gametocytes of Plasmodium vivax, P. ovale⁵⁴	Unable to combat the blood stage
Artemisinin and derivatives	Asexual P. falciparum blood stage parasite and immature gametocyte⁵⁵	Short half-life, demands combination therapy; ineffectual alone
Metronidazole⁵⁶	Trophozoite stage of Entamoeba histolytica⁵⁷⁵⁸	Ineffective against cysts
Nitazoxadine	Trophozoite stage of G. lamblia ⁵⁹⁶⁰	Ineffective against cysts
Pentavalent antimonial	Amastigote stage within macrophages of Leishmania	Unable to combat the insect stage

Lack of selectivity:

Certain anti-protozoal entities are ineffective in differentiating between parasites and host cells, which raises concerns regarding host toxicity.

Table 4: Anti-protozoal drugs exhibiting a lack of selectivity

Drug	Limitations	Impact
Melarsoprol ⁶¹	binds to sulfhydryl groups of both parasite and host enzymes	Reactive encephalopathy in 10 % patients
Metronidazole ⁶²	Might interact with host DNA	Neurotoxicity, GI disturbances
Nifurtimox ⁶³	Emergence of reactive oxygen impairs the host cell	GI disturbances, mutations
Pentamidine ⁶⁴	Influences DNA and mitochondrial activity	Nephrotoxicity, arrhythmia

Cyst form:

It is a dormant stage of the parasite's life cycle, during which it is less prone to drug treatment as it declines metabolic activity.⁶⁵ The cyst wall can impede drugs from penetrating the parasite. Drug-drug interaction and deep residence of certain protozoa and inadequate tissue penetration by drug in an efficacious concentration contribute to PD limitations of anti-protozoal drugs.

PK limitations:

PK is the assessment of what the body does to a drug. Certain antiprotozoal entities demonstrate substantial limitations in this domain and thus exhibit alterations in therapeutic potential.

Minimal oral bioavailability:

Certain antiprotozoal Active pharmaceutical ingredients (API) reveal minimal bioavailability when ingested orally. E.g., Amphotericin B is injected intravenously as it offers minimal bioavailability when given orally. Paromomycin is poorly absorbed, thus revealing minimal systemic outcomes.

Short half-life:

Certain antiprotozoal entities are quickly metabolized and eliminated from body. Nifurtimox has a short half-life, thus requiring a boost in dosing frequency.

Inadequate tissue penetration:

Insufficient distribution to intended site impedes effectiveness in treating antiprotozoal infections. Since pentamidine, which mitigates African trypanosomiasis, demonstrates poor CNS penetration, thus fails to demonstrate maximum therapeutic effects in late-stage African trypanosomiasis.

Drug interaction:

It impedes the efficacy of API and boosts toxicity when one API is co-administered with another.

Classification of Antiprotozoal Drugs 2. Overview of existing drugs for protozoan diseases

Sr.	Drug	Absorption	Distribution	Metabolism	Elimination	Challenges Posed by Adverse Effects/Toxicity	Pharmacodynamics
Anti-Malarial Drugs⁵
	Quinine^6,7	Principally from the upper small intestine, with ease.	Protein bound: 70-95%	Liver mostly CYP3A4 and CYP2C19mediated	8–14 hr. (adults), 6–12 hr. (children). feces and saliva; urine (<5% as unchanged drug)	Hypoglycemia, renal failure, Anginal symptoms, Night blindness, Thrombocytopenia	Parenterally administered quinine treats potentially fatal infections brought on by P. falciparum malaria that is resistant to chloroquine. Quinine has gametocytocidal activity against P. vivax and P. malariae, but it also serves as a blood schizonticide. It is concentrated in P. falciparum's feeding vacuoles due to its weak baseness.
	Chloroquine^8,9	Rapidly and almost completely absorbed from GIT	volume of distribution 200–800 L/kg. Plasma drug concentration is about 55% bound to non-diffusible plasma components.	It is N-dealkylated primarily by CYP2C8 and CYP3A4 to N-desethylchloroquine.	1-2 months. Excretion is quite sluggish, but it is accelerated by urine acidity.	Nerve type deafness; tinnitus, reduced hearing, Skeletal muscle myopathy or neuromyopathy, Stevens-Johnson Syndrome, thrombocytopenia	Chloroquine inhibits the activity of heme polymerase, which causes toxic heme to accumulate in Plasmodium species. It acts for an extended period due to its half-life of 20 to 60 days.
	Amodiaquine ^10,11,12	Rapidly absorbed following oral administration.	Glycoprotein α1-acid binds 90% of the protein.	Cytochrome P450 enzyme CYP2C8 bioactivates it hepatically to produce N-desethylamodiaquine, which is its principal metabolite.	Desethylamodi-aquine is eliminated slowly and has been detected in plasma and blood for up to 1 month after drug administration.	Pyrexia, cough, Asthenia, Abdominal pain Upper respiratory tract infection, Headache, Diarrhea, decreased appetite, Vomiting, Gastroenteritis	Amodiaquine mode of action is yet unknown. 4-Aminoquinolines cause vasodilatation, which lowers blood pressure, depresses the heart muscle, and reduces cardiac conductivity.
	Pyrimethamine ^13,14	Well absorbed with peak levels occurring between 2-6 hr. following administration	Adults: 2.9 L/kg; distributed across liver, spleen, lung, and kidneys. 87% of proteins bind.	Extensively metabolized by the liver	Urine (16% to 32%)	Hematologic AEs occurred across all manifestations.	The therapeutic activity of pyrimethamine, a folic acid antagonist, is explained by differing requirements for nucleic acid precursors needed for growth between host and parasite. This action is very specific to Toxoplasma gondii and plasmodia.
	Proguanil ^1,13	Quickly and effectively absorbed in humans at oral dosages between 50 and 500 mg. Proguanil is taken orally and absorbed gradually.	75% protein-bound	Cytochrome P450 is an enzyme in the liver that processes it to produce cycloguanil, an active triazine metabolite.	Excreted in urine (40–60%) and feces (10%)	Slight hair loss, Mouth ulcers.	Cycloguanil is active metabolite obtained from biguanide derivative proguanil. It works against parasites by preventing the parasitic dihydrofolate reductase enzyme from functioning. It treats acute infection and has preventive and suppressive effects against P. falciparum. It also works well to stifle vivax malaria's clinical assaults.
	Sulfonamides ^1,13,15	Oral absorption of sulphonamides is easy.	Sulfadiazine: widely distributed (including within cerebrospinal fluid), 45–55% protein-bound,	Hepatically metabolized	Sulfamethoxazole and Sulfadiazine: renally excreted.	1-2% of individuals have anorexia, vomiting, nausea, and severe acute haemolytic anaemia. Reactions of hypersensitivity	Dihydropteroate synthetase enzyme inhibitors. Because soluble sulphonamide salts are extremely alkaline and irritating to tissues, parenteral administration is challenging. Pleural, peritoneal, synovial, and ocular fluids all reach high concentrations.
	Mefloquine ¹	Mefloquine is easily absorbed through GI system.	In healthy adults, apparent volume of distribution is wide tissue distribution and is around 20 L/kg. more than 98% of plasma proteins	metabolized in the liver by CYP3A4 enzyme	Excreted in bile and feces	Anxiety disorders, hallucinations, sleep disturbances, psychosis, toxic encephalopathy, convulsions, and delirium. Cardiovascular effects, bradycardia, and sinus arrhythmia.	As a blood schizonticide, it prevents and treats malaria. It Aims at Ribosomelaria 80S. It Aims at the Ribosome 80S
	Atovaquone ¹⁶	Absorption observed using 750 mg tablets.	Atovaquone has a distribution volume of 7.98 L/kg, a high affinity for human serum albumin, and is well bound to plasma protein (>99.5%).	Some evidence suggests limited metabolism (although no metabolites have been identified).	Liver is principal organ involved in drug elimination, while very little (<0.6%) of elimination is accomplished by the kidneys. Over 90% of the amount excreted in bile was in the form of parent drug.	Rash, fever, vomiting, diarrhea, abdominal pain, and headache	Atovaquone's inhibitory action is similar to that of ubiquinone; in atovaquone-responsive parasites, it works by specifically interfering with parallel processes, including pyrimidine biosynthesis and mitochondrial electron transport. Medication atovaquone is quite lipophilic.
	Primaquine ^1,17,18,19	Primaquine is rapidly absorbed in the GIT. Absorption is linear with doses of 15 to 45 mg/kg.	Extensively distributed in tissues, with a mean volume of distribution of 3 L/kg.	Predictably catalyzed by cytochrome P450. Direct glucuronide/ glucose/carbonate/acetate conjugation, hydroxylation, and oxidative deamination of PQ.	Has an elimination half-life of 4-9 hr.	Anorexia, nausea, vomiting, cramps, chest weakness, anemia,	In addition to chloroquine, primaquine is a necessary co-drug for treating all cases of malaria.
	Artemisinin ²⁰	Rapidly and incompletely absorbed after oral administration.	Absolute bioavailability after oral (100 mg/kg) and intravenous (5 mg/kg) administration 12.2.	It is, and all its derivatives metabolize to form the dihydro structural form.	Rapidly eliminated through biotransformation, mostly in liver	Headaches, nausea, vomiting, abnormal bleeding, dark urine, itching and fever.	It is suggested that artemisinin affects nucleic acid and protein metabolism.
Drug acting on leishmaniasis
	Sodium Stibogluconate ^21,22	Administered i.m. or by slow i.v. infusion	0.22 l/kg i.m.	Trivalent antimony is produced by the liver's metabolism of sodium stibogluconate, which could explain why it is harmful when used for an extended period.	Primarily eliminated in urine, with over 80% of the dose being eliminated in less than 6 hr.	GIT complaints (i.e., nausea and diarrhea), musculoskeletal complaints (i.e., muscle and joint pain), fatigue, serum transaminase elevations, T-wave inversion or flattening, pancreatitis, and rarely.	The mode of action of this drug is not clearly understood. When amastigotes are exposed in vitro to 500 mg/ml of pentavalent antimony, levels of purine nucleoside triphosphate, RNA protein, and parasite DNA are reduced by more than 50 %.
	Amphotericin B ^{23-28, 29}	Bioavailability is 100% for intravenous infusion. Amphotericin B's low oral absorption necessitates parenteral administration.	Highly bound (>90%) to plasma proteins. High amounts of amphotericin B are found in tissues such like kidneys, lungs, liver, spleen, and bone marrow.	Not metabolized in the liver. Liposomes engulfed by RES.	Less than 10% of the amount was eliminated over a week, mostly unchanged in urine and feces. D-AMB: urinary excretion (21%); faecal excretion (43%). L-AMB: urinary excretion (5%); faecal excretion (4%).	Nephrotoxicity, nausea, vomiting, rigors, fever, hypertension or hypotension, and hypoxia.	Amphotericin B exhibits a high order of in vitro activity against many species of fungi. H. capsulatum, Coccidioides immitis, Candida species, Blastomyces dermatitidis, Rhodotorula, Cryptococcus neoformans, Sporothrix schenckii, Mucor mucedo, A. fumigatus are all inhibited by concentrations of amphotericin B (0.03 to 1.0 mcg/mL in vitro).
	Miltefosine ^{29, 30}	Oral administration. GI absorption rate in the two-compartment model is estimated to be 0.416 hr-1.	Radioactivity studies have found that miltefosine has a wide distribution with high levels in the kidney, intestinal mucosa, liver, and spleen. Plasma protein binding (96% to 98%).	Metabolized mainly by phospholipase D, releasing choline, choline-containing metabolites, and hexadecanol, which are likely to enter intermediary metabolism.	Entirely removed via phospholipase D's breakdown.	Gastrointestinal effects (vomiting and diarrhea) in 30-50% of patients, Nephrotoxicity, Hepatotoxicity, Teratogenicity	Clinical pharmacodynamics is generally poorly understood, mostly due to the absence of reliable quantitative indicators of parasite burden and treatment response for leishmaniasis.
	Paromomycin ^{29, 31, 32, 33.}	Low bioavailable after oral dose. Due to its absorption limitation, most of the research and clinical trials were performed on paromomycin administered either by i. m. or i. v. route.	Peak plasma concentrations are attained within 1 hr.	Not metabolized	Almost 100% of oral dose is eliminated unchanged in feces; any absorbed drug is gradually excreted in urine.	Nephrotoxicity, Ototoxicity, Hepatotoxicity, Pain at injection site	Binds to ribosomes, resulting in the impediment of protein biosynthesis. A broad-spectrum aminoglycoside antibiotic produced by S. rimosus var. paromomycinus. In vitro and in vivo antibacterial action of paromomycin closely parallels that of neomycin.
	Pentamidine ^{29,34,35,36,37,38}	It is not bioavailable orally owing to the two very basic amidine groups it possesses. It can be given via IV or IM.	Kidney, liver, spleen, adrenal glands. Plasma centration decreased by 32% within10 minutes after the end of infusion, indicating rapid distribution.	Metabolism in human liver microsomes was attributed to CYP2D6 and CYP1A1 enzymes in vitro. The role of CYP1A1 in metabolism of pentamidine was later confirmed in human liver microsomes, with additional contributions from CYP3A5 and CYP4A11; however, CYP2D6 was not found to be involved.	Pentamidine seems to be eliminated in bile, despite the liver's slow release. 24 hr. after IV injection, the liver still contains 99% of absorbed pentamidine. Many investigations documented poor urine excretion in the first 24 hr. after infusion (2.1 to 5.5% or less than 20%).	Acute reactions result from histamine release. A sudden drop in blood pressure, a cardiovascular collapse, nausea, rigidity, fever, injection site pain, and insulin-dependent diabetic mellitus.	Its MOA is not fully understood but believed to perform by interacting with DNA, RNA synthesis, and interfering with mitochondrial enzymes.
Chagas disease:
	Benznidazole ^39,40,41,42	Oral route has nearly complete bioavailability.	Absorbed and then broadly distributed to many tissues, including fetal and placental tissue, at concentrations comparable to those in blood circulation.	While nitro reductases play a role in conversion of the nitroimidazole motif, research has demonstrated that hepatocytes and microsomes from different species have low levels of metabolism and do not generate any human-specific metabolites.	Metabolic and excretion pathways for this drug remain unclear.	At clinically relevant dosages, benznidazole can produce hepatotoxicity, peripheral neuropathy, and angioedema	T. cruzi, the causal agent of Chagas disease, is killed by the trypanocidal drug benznidazole.
	Nifurtimox ^{43,44,45,46,47,48}	After giving a single dosage of 120 mg of nifurtimox with food to adult Chagas patients, mean (%CV) nifurtimox AUC estimates were between 1676-2670 µg√h/L (19–32%), and Cmax estimates ranged between 425-568 µg/L (26–50%), respectively.	It is capable of passing through the blood-brain barrier and reaching the placenta.	Metabolism occurs via nitroreductase enzymes. M-4 and M-6 are two main inactive metabolites that have been found. M-6 is most likely a result of hydrolytic cleavage of hydrazone moiety of nifurtimox, whereas M-4 is a cysteine conjugate of nifurtimox. In human plasma, additional small metabolites have also been found.	In fed condition, the majority of the dosage 44% was retrieved as metabolites in urine. Nifurtimox excretion in the feces and bile has not been investigated.	Most frequently reported adverse reactions (≥5%) are vomiting, abdominal pain, headache, decreased appetite, nausea, pyrexia, and rash.	Nifurtimox holds trypanosomal action against T. cruzi, which mitigates Chagas disease. Additionally, benzofuran entities, such as nifurtimox, suppress parasite dehydrogenase activity.
African Trypanosomiasis (Sleeping Sickness)
	Pentamidine ^49,50,51	Absorbed poorly through GIT and is usually administered parenterally. 4 mg per kg per day Intramuscular/Intravenously for seven to ten days.	Large volume of distribution, poor CNS penetration	Hepatic	Slow renal clearance	Adverse events, including hypotension, nephrotoxic consequences, leukopenia, and hypo- and hyperglycaemia, are frequently linked to treatment.	It's unclear exactly how it works as an antiprotozoal entity. It impedes synthesis of DNA, RNA, phospholipids, and proteins by interfering with nuclear metabolism, according to in vitro tests using mammalian tissues and protozoan Crithidia oncopelti.
	Suramin ^52,53,54,	Initial test dose of 4 to 5 mg/kg of body weight is the normal treatment regimen for suramin. This is followed by five weekly intravenous (i.v.) injections of 20 mg/kg (but not more than 1 g). Poorly absorbed from GIT has poor oral absorption.	Widely distributed to tissues, kidneys, and protein. This drug’s volume of distribution was 31–46 liters.	Suramin is not extensively metabolized.	80% excreted via kidney. With repeated administration, its long half-life (t1/2) of 50 days or more causes drug accumulation.	Nephrotoxicity, hypersensitivity reactions, dermatitis, anemia, peripheral neuropathy, and bone marrow toxicity	Impediment of energy metabolism, blocking surface receptor of paracite.
	Melarsoprol^{55,56,57,58,59,6061}	Current suggested dosage is 2.2 mg/kg per day for 10 days, given slowly by intravenous method; this dose is typically taken orally in conjunction with 1 mg/kg of prednisolone per day.	Trypanocidal effects can be produced at drug concentrations in CSF of 2–20% of plasma levels.	The body rapidly metabolizes medarsoprol to produce a variety of trypanocidal compounds, possibly including melarsen oxide.	Melarsoprol leaves body through unrine and bile.	Peripheral neuropathy, skin rashes, cardio toxicity, and agranulocytosis are among other side effects.	Inhibition of trypanothione metabolism, Impediment in energy production
	Fexinidazole^{62,63,64,65,66,67}	For human African trypanosomiasis brought on by T. brucei gambiense (g-HAT), Fexinidazole is a novel oral therapy.	Apparent volume of distribution is 3222 ± 1199 L.	In vivo, it undergoes significant metabolism to produce two physiologically active metabolites: sulfoxide (M1) and sulfone (M2), which is excreted slowly.	Approximately 0.75–3.15% was recovered in urine over 168 hr., mostly as M1 and M2 metabolites. Elimination is virtually exclusively extra-renal.	Treatment-emergent adverse events (TEAEs) with frequency of >20% included headache (35% of patients), vomiting (28%), sleeplessness (28%), and nausea. reduced appetite (22%), tremor (23%), asthenia (26%), and (21%) with fexinidazole, headache (29%), and vomiting (24%) when using NECT.	It is 5-nitroimidazole that is 2-substituted. Most likely, parasitic nitro-reductases transform it into extremely reactive species, which impair proteins and DNA and eventually kill the parasite. The dosage schedule is made to guarantee that fexinidazole and its reactive metabolites are present at sufficiently high concentration for at least 48 hr. minimum duration of exposure that has been demonstrated in vitro to be effectively trypanocidal.
Toxoplasmosis
	Pyrimethamine^68,69,70	Highly absorbed, reaching peak levels 2-6 hr. after administration. Pyrimethamine is lipid-soluble, easily absorbed from the digestive system, and dispersed throughout the body's cells.	Extensive tissue distribution	Liver is primary site of metabolism.	About 15–30% is eliminated unchanged in urine.	Bone marrow suppression, resulting in anemia, leukopenia, or thrombocytopenia.	When treating simple P. falciparum malaria that is resistant to chloroquine, pyrimethamine is routinely put into use. Therapeutic activity of pyrimethamine, a folic acid antagonist, is explained by differing requirements for nucleic acid precursors needed for growth between host and parasite.

7. Challenges and opportunities in anti-protozoal drug development⁸⁸

A. Challenges:

Drug resistance:

the emergence of drug resistance poses substantial challenges in anti-protozoal drug development. To address these challenges, researchers have been investigating substitute therapeutic options, entailing the usage of benzo[h]chromene derivatives,⁸⁹ antimicrobial peptides⁹⁰ and novel quinoline entities. Preclinical research has demonstrated potential of aforementioned entities, with some exhibiting greater effectiveness against protozoal pathogens than drugs that are currently available. In particular, benzo[h]chromene was revealed to have substantial antiprotozoal potential against Trypanosoma cruzi, with IC values ranging from 19.2 to 68.7µM.⁸⁹ However, antimicrobial peptides hold broad-spectrum efficacy against a variety of protozoal pathogens, especially Plasmodium falciparum, Leishmania infantum, and Trypanosoma brucei.⁹⁰Furthermore, new ferrocenyl-substituted organometallics have been developed and characterized; they hold strong antiprotozoal features and favourable pharmacokinetic aspects.⁹¹ Overall, these results highlight the possibility of novel entities and approaches to address the constraints associated with drug resistance in the development of anti-protozoal therapies.

Complicated life cycles:

Complex life cycles of protozoa present formidable obstacles to the development of anti-protozoal drugs.^92,93 Particularly, the Plasmodium species responsible for malaria possesses a life cycle that entails distinct stages in both the human host and the mosquito vector, making it tough to target with drugs.⁹⁴It is hard to come up with efficient therapies for sleeping sickness owing to the intricate interactions between the parasite and the host's immune system during the Trypanosoma brucei life cycle.⁹⁵

Host-specificity and variability:

A major obstacle in the development of anti-protozoal drugs is host-specificity (the Potential of a parasite to infect and flourish in a certain host).⁹⁶ This concept is pivotal in comprehending the intricate interactions between parasites and their hosts since it has a significant impact on the development of efficient therapeutics. Protozoa frequently exhibit host specificity, necessitating specialized care. Personalized medicine approaches are necessary for better therapy outcomes due to the complexity created by the heterogeneity in parasite genotypes and host immune responses.

Restricted resources for ignored illnesses: Insufficiency in productive and accessible remedies for ailments, viz, Chagas disease, Malaria, and Schistosomiasis, is a cardinal concern, with existing therapies often suffering from constraints like parasite resistance. Breakthrough in the development of anti-protozoan entities depends on resolving this discrepancy. Limited monetary resources along with infrastructure in developing countries, which is a prevalent domain for these ailments, render it harder to run clinical trials and enforce newer therapies.

B. Opportunities for future research and development:

Genomic and proteomic advances:

Prospects for revealing novel therapeutic targets arise from the investigation of protozoan genomes and proteomes. The development of more accurate and potent drugs might be directed by the abundance of data provided by advances in omics technologies.

Collaborative research initiatives:

Progress can be accelerated through strengthened cooperation between public health organizations, pharmaceutical companies, and research institutes. Shared resources and knowledge facilitate a comprehensive approach to addressing challenges in anti-protozoan drug development.

Innovative drug delivery systems:

Exploring novel drug delivery systems can boost the effectiveness of anti-protozoan drugs. Nanotechnology and targeted drug delivery mechanisms provide avenues for improving drug stability, bioavailability, and selective targeting.

5. CONCLUSION:

Protozoal infections are difficult to treat effectively owing to the pharmacodynamics and pharmacokinetics of protozoal medications, as low bioavailability, quick metabolism, and possible toxicity limit their effectiveness to mitigate medical conditions. However, there are encouraging ways to get over these obstacles because to developments in drug design, delivery methods, and molecular changes. The pharmacokinetic profile of protozoal medications can be boosted, leading to better therapeutic outcomes, by investigating prodrug approaches, utilizing targeted delivery mechanisms, and modifying drug formulations. A profound understanding of pharmacokinetics and dynamic limitations is fundamental in the development of effective treatments for protozoal ailments. It not only optimizes drug efficacy but also addresses challenges such as drug resistance, thereby playing a critical role in global health efforts to combat these impactful diseases. In order to treat protozoal illnesses more effectively and safely and eventually improve global health outcomes, additional research in this field is essential.

AUTHORS’ CONTRIBUTION:

ASP: Conceptualisation, intellectual content, and manuscript preparation; MSB: manuscript review.

DECLARATION OF INTEREST STATEMENT:

The authors report there are no competing interests to declare.

ABBREVIATION:

Abbreviation	Meaning
PD	Pharmacodynamic
PK	Pharmacokinetic
T. cruzi	Trypanosoma cruzi
T. brucei	Trypanosoma brucei
P. falciparum	Plasmodium falciparum
P. vivax	Plasmodium vivax
T. dondii	Toxoplasma dondii
L. donovani	Leishmania donovani
C. parvum	Cryptosporidium parvum
G. lamblia	Giardia lamblia
P. ovale	Plasmodium ovale
E. histolytica	Entamoeba histolytica
API	Active pharmaceutical ingredients
GIT	Gastrointestinal track

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Received on 03.09.2025 Revised on 25.10.2025

Accepted on 29.11.2025 Published on 12.02.2026

Available online from February 14, 2026

Res.J. Pharmacology and Pharmacodynamics.2026;18(1):15-28.

DOI: 10.52711/2321-5836.2026.00003

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Creative Commons License.

Drug-drug interaction	Impact
Chloroquine and antiepileptics⁶⁶	Antiepileptics concentration may be elevated with an alteration in chloroquine metabolism
Amodiaquine and a calcium channel blocker ⁶⁷	Serum concentration of Amodiaquine may be ameliorated
Piperaquine and acetaminophen⁶⁸	Piperaquine may ameliorate the hepatotoxic activities of acetaminophen.
Primaquine and amitriptyline ⁶⁹	Ameliorate the risk of irregular heart rhythm and cardiac risk
Mefloquine and acetazolamide⁷⁰	The therapeutic potential of acetazolamide is diminished
Quinine and acetaminophen ⁷¹	Metabolism of acetaminophen is minimal
Quinidine and aceclofenac⁷²	Lowers excretion of quinidine, thus higher serum levels
Proguanil and albendazole ⁷³	Proguanil metabolism is reduced
Chlorproguanil with ambroxol ⁷⁰	Severity of methemoglobinemia can be ameliorated
Pyrimethamine and amitriptyline ⁷⁴	Decreased metabolism of amitriptyline
Sulfadoxine and articaine ⁷⁵	Severity of methemoglobinemia can be ameliorated
Dapsone and acetazolamide ⁷⁶	Therapeutic potential of dapsone can be ameliorated
Sulfamethopyrazine and chloroprocaine ⁷⁷	Therapeutic potential of sulfamethopyrazine can be decreased
Atovaquone and acetylsalicylic acid ⁷⁸	Decreased metabolism of acetylsalicylic acid
Artemisinin and amitriptyline ⁷⁹	Decreased metabolism of artemisinin
Miltefosin and benzocaine ⁸⁰	Severity of methemoglobinemia can be ameliorated
Metronidazole and alcohol ⁸¹	Severe nausea, vomiting, tachycardia
Tinidazole and warfarin	Boosts warfarin impact, thus raising the risk of bleeding
Benznidazole aceclofenac ⁸²	Decreased excretion rate of benznidazole thus ameliorated the serum level
Fexinidazole and acetaminophen ⁸³	Decreased metabolism of acetaminophen
Paromomycin and aceclofenac ⁸⁴	Ameliorated the severity of nephrotoxicity
Antimalarial and quinacrine ⁸⁵	CNS toxicity is entwined with confusion, anxiety, and hallucination
Pentamidine and nephrotoxic drugs ⁸⁶	Raised hurdles of renal impairment
Suramin and capsaicin ⁸⁷	Raised severity of methemoglobinemia