Antimetabolites: Sulfonamides and trimethoprim

Antimetabolites are medications that interfere with the synthesis of DNA.

Some antimetabolites are used in chemotherapy to kill cancer cells, while others are used as antibiotics since they inhibit bacterial folate synthesis.

Folate, or folic acid, also known as vitamin B9, is necessary for the synthesis of nucleic acids, which are the building blocks of DNA and RNA.

Simply put, a lack of folate results in a lack of nucleic acids, which then results in decreased DNA and RNA synthesis, leading to hindered cell division and function.

Now, a key difference between our cells and bacterial cells is that we get all of our folate through our diet, while bacteria can make their own folate from scratch.

Because of this, we can target the bacterial folate synthesis pathway to minimize the damage done to our cells.

So in order to synthesize folate, the bacteria will first use the host’s para-aminobenzoic acid, or pABA, and convert it to dihydropteroic acid via the enzyme dihydropteroate synthetase, or DHPS.

In the second step, dihydropteroic acid is converted into dihydrofolic acid by dihydrofolate synthetase.

The third step is the conversion of dihydrofolic acid into tetrahydrofolic acid via dihydrofolate reductase.

Tetrahydrofolic acid is a folic acid derivative and can be used to synthesize purines like adenine and guanine, which are used to build DNA and RNA, as well as thymidine, which is only used in DNA.

Now, the first group of antimetabolite antibiotics are the sulfonamides, which include sulfamethoxazole, or SMX, sulfisoxazole, and sulfadiazine.

These medications bind to dihydropteroate synthetase, or DHPS, in the first step of folate synthesis and prevents the bacteria from making dihydropteroic acid.

These medications can be given peroral or injected into a vein, but they need to be metabolized by the liver in order to work.

Now, they are broad spectrum and can treat a variety of gram positive and gram negative bacteria, as well as chlamydia and nocardia species.

Next we have trimethoprim, which inhibits the 3rd step of folate synthesis by inhibiting dihydrofolate reductase, or DHFR, preventing the formation of tetrahydrofolic acid.

Now humans also have dihydrofolate reductase, but the bacterial version of this enzyme is 4-5 times more sensitive to this medication.

Trimethoprim is also broad spectrum and is effective against both gram positive and gram negative bacteria.

Now, it’s mainly used in combination with sulfamethoxazole. The combination of these medications is called TMP/SMX.

These medications are used together because they are synergistic and can block folate synthesis at two key steps.

When used alone, both medications are bacteriostatic, meaning they can stop the bacteria from reproducing.

But when combined, they are bactericidal, meaning they will kill off the bacteria.

TMP/SMX is most commonly used to treat traveler's diarrhea and simple urinary tract infections, but it’s also effective in treating pneumonia and sinus infections caused by haemophilus influenzae and moraxella catarrhalis.

It’s the first line therapy for the treatment and prevention of pneumocystis jirovecii infections, which are caused by a yeast-like fungus that can affect immunocompromised people.

Finally, it’s effective against Methicillin Resistant Staph. Aureus, or MRSA.

A few bacteria exist that are notably not susceptible to antimetabolites, and those are Pseudomonas aeruginosa and bacteria from the Mycoplasma family.

Okay for side effects, some people are allergic to sulfonamide and can develop a hypersensitivity reaction to these antibiotics.

They can also develop a cross reaction with other drugs that contain the sulfonamide functional group such as glyburide, a diabetic medication, and thiazide diuretics.