Transuranium elements are the chemical elements with atomic numbers greater than 92, the atomic number of uranium. They are all highly radioactive.
From Asahi Shinbun (12:09AM JST 6/2/2011):
体に入った放射性物質を排出させる薬剤２品が、７月にも医薬品として承認される見通しになった。１日開かれた厚生労働省の薬事・食品衛生審議会の部会で、意見がまとまった。
Two drugs that expel radioactive materials from the body are set to be approved in July, as the experts agreed at a meeting on June 1 of the Committee on Medicine and Food Safety of the Ministry of Health, Labor and Welfare.
２剤は「ジトリペンタートカル」（販売名）と「アエントリペンタート」（同）。どちらも主に点滴薬として使われ、日本メジフィジックス社（東京都）が輸入販売する。
The drugs are " ジトリペンタートカル" and "アエントリペンタート"*. They are used as intravenous drugs. Nihon Mediphysics (in Tokyo) will be the sole importer and distributor of the drugs.
[* I have no idea how they are spelled, or who makes these drugs. These are the brand names to be used in Japan, not the chemical names. The latter has "アエン" in the name, that's "zinc" in Japanese. Does anyone know more about these drugs? What I've found by a quick Google search is at the bottom of the post.]
厚労省によると、原発事故などで放射性物質を大量に吸い込んだり、傷口から入り込んだりしてしまった時に使われる。プルトニウムなどを尿から体外に出す効果が認められているという。２０１０年１０月時点で、米独仏の３カ国で承認されている。
According to the Ministry of Health, Labor and Welfare, the drugs are used for people who have inhaled a large amount of radioactive materials or who have open wounds from which the radioactive materials have entered the body in a nuclear power plant accident. They have been found effective in expelling plutonium in urine. They are approved in the US, Germany and France as of October 2010.
No information is posted on the Ministry's site, and there's nothing in Nihon Mediphysics' site.
CBNews has a bit more information about these two drugs (original in Japanese):
The drugs will help lessen the internal radiation from the transuranium elements - plutonium, americium, and curium - by "replacing calcium and zinc with the transuranium elements" (カルシウムや亜鉛を超ウラン元素で置き換えて) and "expel them in urine".
It seems what these drugs do is to chilate the transuranium elements.
So I googled "plutonium chilation" and the search came up with this paper "Three Plutonium Chelation Cases At Los Alamos National Laboratory" (Oct 2010) (emphasis is mine):
Abstract
Chelation treatments with dosages of 1 g of either Ca-DTPA (Trisodium calcium diethylenetriaminepentaacetate) or Zn-DTPA (Trisodium zinc diethylenetriaminepentaacetate) Occupational Medicine in three recent cases of wounds contaminated with metallic forms of 239Pu. All cases were finger punctures, and each were undertaken at Los Alamoschelation injection contained the same dosage of DTPA. One subject was treated only once, while the other two received multiple injections. Additional measurements of wound, urine, and excised tissues were taken for one of the cases. These additional measurements served to improve the estimate of the efficacy of the chelation treatment. The efficacy of the chelation treatments was compared for the three cases. Results were interpreted using models, and useful heuristics for estimating the intake amount and final committed doses were presented. In spite of significant differences in the treatments and in the estimated intake amounts and doses amongst the three cases, a difference of four orders of magnitude was observed between the highest excretion data point and the values observed at about 100 d for all cases. Differences between efficacies of Zn-DTPA and Ca-DTPA could not be observed in this study. An efficacy factor of about 50 was observed for a chelation treatment, which was administered at about 1.5 y after the incident, though the corresponding averted dose was very small (LA-UR 09-02934).
I don't know if these two drugs mentioned in the abstract of the paper above are the ones to be sold in Japan, but the chemical names suggest they are.