Up for Grabs may ostensibly be about high art but it sinks pretty low to prove it.

Aspiring art dealer Loren's score of a lifetime has turned into a nightmare. She's had to guarantee she'll sell her client's Jackson Pollock for an inflated £20 million and risks losing home and husband, let alone commission, if she doesn't. As a result, she's willing to do just about anything to pry open the chequebooks of three potential buyers - including having lesbian sex with one while watched by her husband, unable to participate after a botched penile enlargement operation, and enabling another, an overweight, homophobic businessman, to 'take it up the ass' with an enormous dildo.

Any plot synopsis, of course, is beside the point, because the draw for Up for Grabs' UK premiere has nothing to do with the play and everything to do with its casting. In her performance as Loren, Madonna (billed as Madonna Ritchie in the programme) - making her West End debut - must be given an A for effort. It's not an easy role, nor a small one. She's onstage the entire time, lurking about even when other characters take the limelight. She's not terrible either: she doesn't flub her lines too often or fall over and she looks fantastic.

But neither is Madonna much good. She can't project and at points, especially during the opening monologue, is almost inaudible. She also seems to mistake motion for emotion, buckling her knees and flailing her arms but rarely expressing much and certainly not the fear and desperation required of Loren. "It's terrifying," she says while crunching a mint, her face an unconvincing blank.

Luckily, there's some able support, most notably in the form of Daniel Pino as the coked-up but incapacitated dotcom millionaire and Sian Thomas as the prim English art consultant who lets her hair down. Jeremy Herbert's back-projected, sliding-room set looks ultra-modern, but it's cold and overly complicated (technical difficulties caused the cancellation of three previews and, on the night I attended, the set broke down, prompting an extra 20-minute interval). And Laurence Boswell's direction never seems to quite gel, dragging in some places and spluttering in others.

However, this - as well as the performers' apparent unease - may have less to do with Boswell's lack of skill than with the evening's biggest disappointment. The audience, or rather the contingent whose reactions bordered on the hysterical. Whooping and cheering throughout (and drowning out innumerable lines of dialogue), peering up her nose and blouse with opera glasses (even from the vantagepoint of the front of the stalls), snapping a stream of blinding flash photographs (despite bag checks on the way in), stampeding over one another to get closer at the curtain call and even bellowing Madonna's name during quiet scenes of crisis for her character. Ironically, by such behaviour, these die-hard fans show their idol extreme disrespect.

Small wonder the lady looks so grim when she takes her bow; I'm more embarrassed for her fans than for her.

- Terri Paddock