Chapter 9 in Radiobiology for the Radiobiologist is the best source I could find for this topic

Radioprotectors are agents that, when given before or during radiation, reduce the likelihood of early and/or late effects from developing.

Types of Radioprotectors

Most radioprotectors are sulfhydryl compounds. They contain a sulfhydrl group (S-H) attached to a short carbon chain with a reactive compound at the opposite end.
The example from Hall et all is cysteamine:

\begin{align} \text{SH}-\text{CH}_2-\text{CH}_2-\text{NH}_2 \end{align}

These agents are only effect on radiation types that require oxygen for 'fixing' - ie. that function through indirect action of radiation.

Mechanisms and Examples

Scavenging of Free Radicals

The sulfhydryl group may act by chemically reacting with free radicals generated by indirectly ionising radiation and preventing their interaction with DNA

Improvement of DNA Repair

The sulhydryl group may donate a hydrogen atom to assist in DNA repair pathways.

Useful radioprotectors

Amifostine (WR-2721) is the only clinically used radioprotector. It is a prodrug that is converted with the cytoplasm of cells to its active form WR-1065. It does not suffer from the toxicities seen with the early radioprotectors cysteine and cysteamine. A few centres use it to reduce early and late effects in head and neck treatments, and it is carried by astronauts in space.

Clinical use of Radioprotectors

Although amifostine has been shown to subjectively improve patient symptoms in head and neck treatment, with no reduction in local control, it remains suspect due to concerns over tumour protection. Animal studies suggest that uptake into normal cells is faster than tumour cells.

Dose Reduction Factor

Similar to the Dose Enchancement Factor, the Dose Reduction Factor is used to describe the effectiveness of a radioprotector:

\begin{align} \text{DRF}=\frac{\text{Dose of radiation in presence of drug}}{\text{Dose of radiation in absence of drug}}\text{for same lethality} \end{align}