New contraceptive found in shellfish

A sugar derived molecule from the shells of shellfish has shown promise as a method of birth control
02 December 2022

Interview with 

Bill Colledge, University of Cambridge


Mussels on the seashore


A new contraceptive inspired by Roman history and shellfish is being investigated by scientists in Sweden. Many women currently use the birth control pill, but some would prefer to prevent pregnancy without altering their hormones. Ulrike Schimpf has been developing a way to use the substance chitosan - a sugar-based molecule that comes from the outer skeletons of animals like shellfish - to reinforce the natural mucus barrier which forms in a woman’s cervix and can temporarily block the passage of sperm. Professor of Reproductive Physiology at the University of Cambridge, Bill Colledge, has been looking at the study results for us…

Bill - They have effectively developed a barrier method to prevent the sperm from reaching the egg and causing fertilisation. And they've used a substance called chitosan, which is a very long sugar molecule. And it's actually derived from chitin, which is present in the exoskeleton of insects and crusta. And what the substance does is it binds to mucus within the female reproductive tract and it effectively plugs the entry of the cervix. But interestingly, it's been known for a long time, the Romans used to use half a lemon placed up against the cervix as a type of diaphragm to block the penetration of sperm. And they use this as a contraceptive.

James - What is it about the chitosans that make them so fit for this purpose?

Bill - They're natural substances and they've been used in a variety of different applications, including clinically. So there's a lot known about their safety and efficacy. The mucus within the vagina does change throughout the menstrual cycle, and some people use this as a normal form of contraception. They monitor the mucus, it's called the billings method. And what women can do is they can monitor the viscosity of the mucus throughout the cycle. And at the time when they're ovulating, the viscosity changes and it becomes more sort of like it's described as egg white. And at this time the sperm can actually penetrate through the mucus and can get through the cervix and can reach the egg. So effectively what this study is doing is adding something into the vagina that will bind to the mucus, change its thickness, and therefore stop the sperm from penetrating.

James - You mentioned that this is a barrier method of contraception. I wonder if I could ask you the benefits of barrier methods over different methods of contraception, more hormonal based ones?

Bill - It's going to have less side effects. So women take the contraceptive pill, which is either progesterone or progesterone, a little bit of estrogen, mood changes and things like this. Whereas this is not going to cause that there's no hormones involved.

James - I mean, it's all sounding very promising. Is there anything that you notice that might halt our enthusiasm?

Bill - Well, the authors discuss the various caveats and the limitations of the study. Obviously it's very early stages. One of the main caveats is that the work that they did was done using a female sheep as a model, and the vagina and the cervix of the sheep are slightly different from the humans. So there may be some differences when they tried to translate it into doing it in women. They also point out that they haven't taken into account the effects of sexual intercourse, which is a very physical thing, so that might limit its efficacy. The other thing that they don't look at is how long it lasts. So they administer it and one hour later they show the effects. And I think the effects certainly last four hours, which is the limit of the study, but they don't know how long it'll last thereafter. So I think they're probably going to, if you use this, you may have to use it prior to any sexual intercourse, every time.

James - And a 98% efficacy rate. Could you contextualize that against other contraceptive methods?

Bill - The hormonal contraceptive methods are very effective. They're up in the 90% if used correctly, so it's as effective. Of course, they've only shown that it blocks sperm. They haven't looked to see whether it affects the ability to get pregnant. So although 98% sounds very, very effective, you're still gonna get 2% of the sperm penetrating. And of course, you only need one sperm and one egg to result in fertility. So until they actually check what effect it has on fertility and live births, we don't know whether that'll be sufficient. But by analogy to the hormonal contraceptives, which are of a similar sort of efficacy, you would think it would be as effective.


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