Please answer the following question: Information:  - Antifolates are a class of antimetabolite medications that antagonise (that is, block) the actions of folic acid (vitamin B). Folic acid's primary function in the body is as a cofactor to various methyltransferases involved in serine, methionine, thymidine and purine biosynthesis. Consequently, antifolates inhibit cell division, DNA/RNA synthesis and repair and protein synthesis. Some such as proguanil, pyrimethamine and trimethoprim selectively inhibit folate's actions in microbial organisms such as bacteria, protozoa and fungi. The majority of antifolates work by inhibiting dihydrofolate reductase (DHFR).  - Cycloguanil is a dihydrofolate reductase inhibitor , and is a metabolite of the antimalarial drug proguanil ; its formation in vivo has been thought to be primarily responsible for the antimalarial activity of proguanil . However , more recent work has indicated that , while proguanil is synergistic with the drug atovaquone ( as in the combination Malarone ) , cycloguanil is in fact antagonistic to the effects of atovaquone , suggesting that , unlike cycloguanil , proguanil may have an alternative mechanism of antimalarial action besides dihydrofolate reductase inhibition . Although cycloguanil is not currently in general use as an antimalarial , the continuing development of resistance to current antimalarial drugs has led to renewed interest in studying the use of cycloguanil in combination with other drugs .  - Dihydrofolate reductase, or DHFR, is an enzyme that reduces dihydrofolic acid to tetrahydrofolic acid, using NADPH as electron donor, which can be converted to the kinds of tetrahydrofolate cofactors used in 1-carbon transfer chemistry. In humans, the DHFR enzyme is encoded by the "DHFR" gene. It is found in the q11q22 region of chromosome 5. Bacterial species possess distinct DHFR enzymes (based on their pattern of binding diaminoheterocyclic molecules), but mammalian DHFRs are highly similar.  - A dihydrofolate reductase inhibitor (DHFR inhibitor) is a molecule that inhibits the function of dihydrofolate reductase, and is a type of antifolate.  - Atovaquone (alternative spelling: atavaquone) is a chemical compound that belongs to the class of naphthoquinones. Atovaquone is a hydroxy-1,4-naphthoquinone, an analog of ubiquinone, with antipneumocystic activity. It is manufactured in the US in the liquid form, or oral suspension, under the brand name Mepron.    After reading the paragraphs above, we are interested in knowing the entity with which 'cycloguanil' exhibits the relationship of 'instance of'. Find the answer from the choices below.  Choices: - activity  - brand  - chemical compound  - class  - dna  - electron  - enzyme  - function  - gene  - humans  - liquid  - molecule  - species  - vitamin  - work
Answer:
chemical compound